Snape was in the Hogwarts library the next evening, heading along the paved centre aisle on his way through the study tables to the reference section. Beneath one arm were books he'd already selected, as well as rolls of parchment and quills.

An earlier visit to the Archive had revealed that room to be dark and cold and decidedly lacking in any form of Charity, irrespective of how long he'd spent standing beside the be-candled table trying to summon her. He wasn't sure if this was a good thing, or a very bad thing indeed. And so he'd come to the library with several intents: the first was to reconsider the book Madam Pince had found for him explaining ghosts in limbo and abhumans. He'd abandoned it fearfully too soon last time, and felt sure there may be more information he needed to know.

His other mission was to cross-reference some theories on a cure for Crone's Decline, as promised for the Wicce, or, more importantly, to restore the beautiful mind that he watched fade a little each day. Like a dying star Sinistra had once described, its rays, its light, no longer reached into the farthest corners of the universe. He had to help McGonagall.

Focussed as he was, he almost walked past a dark-haired boy in Slytherin uniform sitting at the end of a study table, the light of an arced lamp beating down on the large volume before him, using a pencil to underline paragraphs of text. Snape's years as Head of House had trained him to spot a Slytherin transgressing at a thousand paces, and defacing one of Madam Pince's library books was about as serious a crime a young serpent could commit.

He halted near the absorbed boy and for a moment drew the shocked impression it was Servius – something about the line of the silhouette had a stark resemblance – then realised it was Wait for William. Collecting himself, he cleared his throat and said: "Your Dossier will report that, Huan. Madam Pince will not let you out alive."

William jumped a clear foot in his seat and gazed up with startled eyes. "S-sorry sir! I, uh, I have an eraser -,"

Snape withdrew his wand and cleared the markings off the page. "Don't. Do it. Again." As William nodded, Snape's mind was swiftly cast back to the conversation in Potter's office and he examined the boy with a slightly more critical eye. "How is your father?"

"Dad?" said William slightly confused and sitting upright. "He's…uh…he's fine I guess. I don't hear much from him."

"Still in America, is he?"

"Yes sir. At the University."

"And your…mother?"

"I don't have a mother sir!"

"You mean you don't know who she is."

"No sir."

Snape held his gaze a moment longer in thought, then he glanced at the empty table where William was sitting. "You're here alone?"

"Yes sir. I'm…I'm trying to earn some House points sir."

"Indeed? How so?"

"Extra credit assignment sir. Astronomy. There's a Black Moon at the end of the week and Professor Sinistra told us to research the traditions and rituals that go with it. Did you know sir that it's supposed to make wizarding folk pine?"

"Pine?"

"Yes sir. We're supposed to feel sad on a Black Moon sir. Have you noticed that?"

"Not especially," replied Snape, off hand. There was a chart on the page of William's book and he swivelled it slightly to look closer. "A lunar calendar."

"That's right, sir," said William, warming to the idea of a slightly-friendlier-than-usual chat with his hero, the Potions Master. "There. That's the Black Moon. They're quite rare."

"At the end of this week did you say?"

"The thirty-first. The first new moon was on the first of January."

Snape flicked the page and scanned it speedily, noting the images of druids brewing and Minotaurs walking on water. At the bottom of the page was a graphic of pale, hairless and naked beings, woebegone and wild-eyed, appearing to emerge from a tunnel. Snape pointed to them. "Who are they? Pining wizards?"

William glanced up at him, then smiled, delighted to share his new-found knowledge. "No sir, they're ghosts and lost souls. Apparently a Black Moon is the best time to summon them."

There was a tinking noise at the window and Servius knew exactly what it was. He was lying on his bed in Slughorn's house, flicking through the homework assignments that had been sent to his Hog Doss through the week and wondering whether to bother doing any of them. He'd be gone in a week for sure, thanks to the awful investigation with Mr Credge. Servius had been so constricted with remorse at times he'd been unable to speak, and Mr Credge had interpreted that as either an inability or a refusal to explain. Professor Sinistra had held Servius' hand and squeezed it encouragingly, doing her best to reassure Mr Credge that Servius was simply too emotional to do an interview justice, but her anxiety was obvious.

There was some truth to her statement, however. Here he was, only hours away from a verdict he'd craved since taking a seat on the Hogwarts Express, and dreading it. He didn't want to go home, rather, he didn't want to go back to Trowbridge. This hazy realisation had dawned fully the evening Slughorn had escorted him to his cottage in Hogsmeade, as they trudged along the winding lanes through feathery snow, his father's blood still matting his hair. They had paused outside the golden glow of Honeydukes and Servius had thought he didn't have an appetite, but at the scent of butter and sugar wafting out the door as it opened to rosy-cheeked children clutching their bounties, Servius' tummy told him better. He'd never been to Honeydukes and gazed longingly through the bow-windowed shopfront, the sight enough to numb his mind to anything else.

Slughorn had chuckled and pocketed him a few Galleons and afterwards, his mouth full of toffees as they ambled up the hill, he said to Slughorn, "A man called Harry Potter told me that he'd once ridden on a Hippogriff."

"Ho ho, did he indeed? Well Harry was exceedingly skilled at getting away with things."

"So it's possible? You can ride a Hippogriff?"

"You wouldn't catch me on one, but I have heard it's done. You should ask your father about that," and then Slughorn had seemed to catch himself too late, and in a consternation he pulled out his fob-watch but forgot to flip it open. "Yes, well, maybe one day, eh?"

And as they arrived outside Slughorn's cottage, his Christmas wreath still affixed to the front of his door, a carriage-lamp streetlight blinked on and Servius had burst into tears.

There was another tinking sound and Servius jumped up from the bed and ran to the window, glancing quickly outside before throwing up the sash. Amelie's origami moth fluttered through and began tapping Servius lightly about the head before he caught it and shut the window against the cold again. He opened the message where he stood and read: Professor Longbottom found by your father and Harry Potter! Taken to hospital. My father says he is under a very strong Imperious Curse but that they might be able to break it. Now they know you were telling the truth!

He read the note several times but it raised more questions than it answered. Was the Longbottom they'd found the same one as Christmas Day Rabastan? Was he the one that had worn the mask in the Forbidden Forest? Was an Imperious Curse the magic that enabled two men to be the same? And then there was Amelie's final exclamation; was it communicating happiness or alarm? Did Servius' validation help his case or not, and where did Amelie even stand on the matter? Her promise had been to help him get expelled, but now, the thought of going to school in Trowbridge and never seeing her again made his heart want to wrestle right out of his chest.

He ripped out a page of his Dossier and wrote a message asking Amelie to meet him at the Shrieking Shack – the name he'd readily adopted since discovering that the Shack was in fact part of Hogwarts lore – after school during free time. Then he folded his paper aeroplane and opened the window again to launch it. The Hellmann's residence was only a few houses away from Slughorn's, and from where he was standing he could see a flashing light coming from her room: a Morse code to let him know she was there. The plane was soon flying smoothly over the rooftops and a minute later he saw the three blinks of her lit wand to let him know she had accio'd it to her. He lowered the window slowly, feeling the blood-bond flowing freely in his veins, wondering if tomorrow might be the last time he'd see her.

Snape was in his office, his now tattered and limp piece of charmed parchment and quill lying on the desk before him. He had written to Charity hours earlier, and the message had been Conveyed, but no reply was forthcoming. Every few minutes he checked it, and to keep himself busy he read the tome he'd found in the library, but it was handwritten and in Middle English and extremely taxing. So he welcomed the distraction when there was a knock at his door.

He opened it with his wand, expecting a fifth-year with complaints about the potions assignment he'd issued only half an hour earlier, but it was Sinistra who stepped inside and gently shut the door behind her. He jumped to his feet.

She looked unhappy and his smile faltered. "Severus, I'm afraid the investigation interview with Servius didn't go very well."

"It didn't?"

"No. Servius was…overwhelmed. With remorse, I think. Mr Credge bombarded him with questions and he just…sort of…failed…"

Snape processed this a moment. "He went inside."

"Pardon?"

"I used to do it. All the time at his age, when it got too much. He went inside and couldn't come out. There would have been words, things he wanted to say, but he would have been trapped in the walls of his own head." He shook his head and swore. "Is he alright?"

"I'm sorry I couldn't come sooner -,"

"No, I understand, thank you for being there."

"I think this time he's in terrible shock."

Snape bristled. "I know what Credge said about non-interference, but he's my son and I want to see him. I'll go to Hogsmeade tonight."

"No, Severus, don't make it worse for him, don't jeopardise the process," replied Sinistra with urgency. "I told Credge that Servius was too emotional to respond properly, he's agreed to consider that, he said he may give Servius a second opportunity. Send him a Patronus instead."

"I've sent a Patronus every day! He can't reply, I have no idea what he's thinking -,"

"That doesn't matter. He just needs to know you're there, that you care, that you forgive him. Did you say that? Did you say you forgive him?"

"Yes. Of course. I never got a moment alone with him after the Shack, but I told him in a Patronus."

"Did you tell him you love him?" asked Sinistra, and the words sounded pinched, as if in effort.

Snape opened his mouth and then closed it again, and his eyes lowered. He picked up the piece of parchment on the table then dropped it listlessly. "No," he admitted, almost inaudibly. "No. I haven't said that."

She took two steps towards him. "Why not? Why haven't you said that?"

He swallowed and his eyes could only meet hers briefly. "It just seems…unnecessary. He knows."

"Are you sure?"

"He's never said it to me. You don't need to say it. It's what you do that counts -,"

"No Severus. You need to say it. You need to cuddle him and tell him you love him. He's a child."

"A child? He's almost my height, has a girlfriend and can cast dark magic!"

She smiled. "He's twelve. And sometimes grownups need to hear it too. Everyone wants to hear it." She came to his desk, laid her fingertips on the edge of it and paused until he looked at her. "Everyone wants to hear I love you."

Did he love her? She was gazing at him steadily, the faint smile still lingering, but no words came to his head, nothing poured out the way they had with Charity and yet he knew he could trust Sinistra with his life. Her support – of him, of Servius – had been unwavering. And she was beautiful: his mind turned to how she'd felt beneath his fingers, with his lips, the taste of her, and hardly conscious of doing it, he boldly stepped towards her and cupped her face in his hands before kissing her hard on the lips.

She broke apart. "What?" she murmured, searching his eyes, confused but hopeful. "I don't understand…"

"If he leaves, if Servius is expelled and…I must leave Hogwarts, then…I don't know what will become of us. You and me. Perhaps there are choices…it is still too uncertain…but the only words I can give you now are less than what you deserve. They are weak, they're torn, they're not what I want to give you."

She moved back, hurt and dismay plain on her face before she seemed to almost sway on the spot. She lowered herself into one of his visitor chairs and he saw how she paled, only two spots of red in each cheek betraying the fevered rushing of blood within her. "Are you still unwell?" he asked gravely. "This seems to be going on some time -,"

"I'm not unwell…I mean, I…can we…can we please talk?"

"A glass of water?"

"Severus – please - sit down."

He nodded and returned to his side of the desk. He was just about to sit when there came a hailing from his fireplace, and McGonagall's voice was heard.

"Severus, are you there?"

"Ma'am."

"I'm sorry this is late, but can you come immediately to my office? It's Harry. He needs to speak to us about Neville."

He glanced at Sinistra, but she'd closed her eyes and bowed her head. He swallowed. "Um…yes Ma'am, of course, I'll be with you directly."

He turned to Sinistra apologetically, but she was waving her hand. "Go. Go. We can talk later."

"You remain in here as long as you need," he murmured. "Perhaps I'll be back in ten minutes."

"Thank you. I'll just take a moment."

And she was silent as he crossed the floor to his door, which he pulled shut quietly behind him, but not before he heard what sounded like a gasping sob.

McGonagall had laid out a tea service on her side table and was already seated in one of the armchairs when he arrived. She beckoned him in and indicated the empty chair, then she said, apparently to the room: "Professor Snape is here, Harry – will you come through?"

"Oh, uh, it's just I'm at home -," replied the disembodied voice of Potter.

McGonagall arched a brow. "Of course. Then carry on."

At that point, a child's hiccoughey crying was heard in the background of the Floo connection and Potter fell silent and McGonagall and Snape exchanged looks.

"Perhaps this is a bad time, Harry?"

Meowing could be distinctly heard. More crying. Then Potter was muttering under his breath.

McGonagall coughed politely. "Harry – the Floo is still open -,"

"I'm coming through," he replied rather sharply, and there was a flurry in the fireplace as he arrived in a swirl of green, wearing a heavy scowl. He dusted himself off. Obviously he'd been intending to relax at home for the evening as he'd replaced his MoM office uniform with a pair of baggy trackpants, trainers and was zipping up a Gryffindor Quidditch jacket to hide the stained t-shirt underneath. "Honestly," he said, "it's like they know I'm trying to have an important conversation! The minute I open a Floo connection, they won't leave me alone!"

Dumbledore was chuckling, and McGonagall worked hard at suppressing a smile. "We won't take long, I'm sure. Tea?"

"Ta. I'm gasping."

Once Potter was seated, he took a slurp of tea, crossed his legs and began. "Neville's in the Janus Thickey ward at St Mungos. It's not for his health: he was very dehydrated and hungry when we found him, but has been otherwise pronounced in good shape. He's well nourished, his hair is trimmed, his nails are well kept, clean-shaven. As recently as only a week ago I'd say Neville had been walking and talking around Hogwarts, eating in the Great Hall and teaching in the greenhouses."

"Well then no wonder we didn't suspect anything," said McGonagall. "Then what was all the Polyjuice for?"

"Professor, he wasn't himself at all. Rabastan Lestrange has used a combination of Imperious Curse and Polyjuice impersonation to infiltrate the school and grounds. For how long I can't exactly say."

"Is he still under the curse?" asked Snape.

"The Curse-Breakers are with him and are struggling with it, but they have come across this kind before. It's not your standard Imperious. It is powerful and very resistant."

"There was a wizard…found wandering in the woods…" murmured Snape and Potter nodded enthusiastically.

"You've heard about it? As long as five years they think. Rabastan's clearly been moving in some dark circles since the war."

"But why Neville of all people?" asked Dumbledore. "And what's this I hear about him having the Dark Mark? Was he a Death Eater?"

Snape shook his head. "He never went to any Death Eater meetings I attended. Could he have been initiated after the war?"

"We'll know more for certain when we've cracked the curse," said Potter. "But I have some theories based on history that made Neville and Rabastan intersect. Rabastan was integral to the torture that drove Neville's parents to madness, along with Barty Crouch jnr. Those two were friends. Neville was the unknown quantity in the Prophesy – the other boy born at the end of July – Voldemort couldn't let that matter rest. What if he'd invested all his energies defeating me, only for the true Chosen One to be Neville?"

"Did he plan to kill Neville?" said McGonagall alarmed.

"No. On the contrary, he wanted Neville alive. Alive and close."

"But only one could live!" said Dumbledore.

"Unless he made Neville a Horcrux," said Snape, instantly seeing where Potter was headed. He'd been pondering on similar theories of his own. "Do by design what he did by accident with Potter. He set out to change destiny, to prove the prophesy wrong. That's why he wanted it so badly."

"So Neville was forced to become a Death Eater? Does it work if it's involuntary?" asked McGonagall.

"Rabastan ran the tortures here at Hogwarts, along with the Carrows," said Snape, his voice laced with bitterness. "I expect he would have branded him then. And started the Imperious curse immediately. Longbottom's personality changed. I thought he was growing confident…"

Potter nodded. "Voldemort was having a bit of an in-joke when he invited Neville to join them. Neville already was a Death Eater."

"I don't believe it," said McGonagall, shocked, her teacup trembling. "I saw him cut off Nagini's head with my own eyes."

"I told him to do it," said Potter. "And he had to comply or give away his true status. Besides, Voldemort didn't need Nagini anymore, and its likely Neville knew Nagini was disposable. Neville was the best new Horcrux, just without the protective bubble – unless you count Dumbledore's Army, of course. Hidden in plain sight."

"And since he had Neville as his inside man, he didn't need me anymore," said Snape. "It was a straightforward decision to kill me for ownership of the Elder Wand. With Longbottom and the wand, nothing else was necessary. Longbottom was in much greater proximity to Potter than I was, and to the Order by then."

"But Neville can't have been made a Horcrux because Voldemort is dead!" said Dumbledore.

"But Potter was the true owner of the Elder Wand," said Snape. "We assumed that was all that was necessary to defeat him since Potter had died already and we believed all the Horcruxes were then destroyed. We believed the Dark Lord to be dead - I'm not sure Rabastan does, and perhaps he's right."

"So Rabastan…thinks… Voldemort is going return?" said McGonagall, looking slight queasy.

"My theory is Rabastan is – was - keeping Neville under control and nearby for Voldemort," said Potter solemnly.

Snape stared at him, nodding slowly. "Do you think he has a repeat of Quirrell planned? Voldemort's own Horcrux to host him?"

But Potter shrugged and shook his head. "I think he was waiting for something. A time, a place where Voldemort and Neville could be reunited. Then who knows what kind of unholy parasitic arrangement would be magicked up."

"But if Longbottom was so important to Voldemort, why leave him to be found?" asked McGonagall, wide eyed. "I mean, a few days longer in that pit and he could have died!"

"It's possible Neville has outlived his usefulness," answered Potter. "Maybe Rabastan's using Neville to throw us off the scent, a red herring. What with the caterwaul, the Foe-glass – he obviously knows now that we have Neville. But I think he has someone better for Voldemort now, someone even better than a Horcrux. Someone who could provide all the blood and sustenance he needs to be as strong as he ever was. His own. Delphini. They just had to wait until she was old enough to know she wasn't a squib."

Better to raise strong children than repair broken men, thought Snape with bitter irony. "I know when," he murmured. "I know what he's been waiting for. And what he thinks he needs to make it happen."

"The Resurrection Stone?" asked Potter.

"When? Where?" said McGonagall and Dumbledore.

"Tomorrow, in fact," said Snape. "It's a Black Moon."

McGonagall looked to Dumbledore with alarmed eyes, but Potter was puzzled.

"What does that mean?"

"The Black Moon is a sacred date when ghosts and lost souls can be summoned. The Centaurs save one lost soul using the Resurrection Stone each Black Moon, before the abhumans can get them. The abhumans can roam freely after that, taking what they can from limbo into hell. But only for that phase of the moon."

There was a moment of silence as three pairs of eyes stared at him, in equal parts confounded and horrified.

"What's an abhuman?" asked Potter in a small voice.

"A little like the Inferi," Dumbledore told him. "But they don't answer to wizards…they belong to something far, far darker."

"Merlin's pearly whites," muttered Potter, shaking his head. "And this is happening tomorrow?"

"That's why Rabastan fled," said Snape. "He knew the game was up and left us Longbottom to throw us off. Now he'll just return for what he needs to do, and as far as he's concerned, he's the next Peter Pettigrew."

Dumbledore coughed for attention. "But you said he needs the Resurrection Stone. That's long gone…isn't it Harry?"

Both Potter and Snape shook their heads. "He learnt about the existence of the stone from Neville under the curse. And Neville knew because…because I told him. It was when we were Aurors."

"He's found it?"

"The Centaurs found it first. They've returned it to its rightful place," said Snape. "Rabastan's just connected the dots, and while he did so, Longbottom provided the perfect cover for him to search for the stone unsuspected. But if Rabastan removes it, then devastation will follow. I don't know what or how, but I suspect the abhumans may have role to play."

McGonagall suddenly put down her teacup and stood. "Mother of Merlin, Severus, I can barely stand to listen to this any further. I'm struggling to believe it – if it were anyone other than you and Harry -,"

"Professor," said Potter, "if Professor Snape has correctly guessed Rabastan's plan, and it makes sense, then I am going to need to be here tomorrow with my team. He can't find out. We have to ambush him."

"Well…well of course," she said, her colour rising. "You must do what you need to do. I shan't stand in your way. I will ask Hagrid to let the Centaurs know as well." She then turned to Snape and lifted her chin. "It would appear I owe you and Servius an apology. He was telling the truth and you both attempted to warn me about Neville – I mean, Rabastan. I stand corrected; I'm sorry I failed to hear you. But that doesn't have any bearing on the current investigation - ,"

"Well…it might do," said Potter.

McGonagall turned to him with pointedly raised brows.

"Perhaps Servius wouldn't have run off and been so…so rash if he'd thought you'd taken him seriously."

When McGonagall pursed her lips, Potter brazenly continued. "I mean, I for one know how it feels to think no-one seems to get what's going on but you. It's extremely demoralising to carry a weight alone which you can't do anything about, but the people who are supposed to trust you and look after you are letting you down again and again -,"

"Thank you Harry -,"

"- and you have to find some way to just believe in yourself, which is really hard to do when you're only eleven or twelve -,"

"Yes, thank you Harry -,"

"– and everyone around you is laughing or - or taking the piss, and you're also trying to adjust to this crazy new life -,"

"Quite so, thank you -,"

"- and some tosser is also bullying you and making you question yourself at the same time -,"

Snape frowned and studied the floor.

" – and then you come across a - a book – some unnamed book with a spell in it that may be an answer - and in a moment of desperation you think: it clearly worked for that kid, you know, its intended for enemies… and so you try it! You're desperate and directionless and you're at that point with nothing left to lose..."

Potter stopped, only just becoming aware of the stunned silence in the room. He chewed his lip.

"I think it would be fair to say we've all had a time in our life like that," murmured Dumbledore after what felt like an hour.

Nigellus spoke up. "Investigations indeed! Bureaucratic nonsense. The lad was simply experimenting and Snape got in the way of his own curse. Taking risks and exploring is what life is all about at Hogwarts. If Servius didn't mean it and said he was sorry, then it's simply boys being boys. Get him back to school and concentrate on catching the real criminal."

Reflective quiet once more. Then Potter got a little awkwardly to his feet and said, "I, uh, really should be getting back -,"

"Thank you so much for the briefing, Harry," said McGonagall, who'd obviously taken the various degrees of feedback to heart and now looked wan and dejected. With her fingers at her brooch, she sank back down in her seat. "Do let me know how things progress tomorrow, and if there's anything I need to do."

"Yeah, uh, absolutely. I'll, um, for what it's worth – I'm dropping the civil process against Servius. My Department have signed the matter over to the capable hands of the Board. Without charges being laid, I'm prepared to accept Professor Snape's argument that a precedent has been set. Making an example of Servius doesn't help Servius – and I think that's what he really needs."

Snape raised his eyes to Potter and nodded, and the boy who lived stepped into the fireplace and disappeared.

The tip of Servius' tongue was unconsciously poking out the corner of his mouth as he concentrated on this most vital and importunate of tasks: graffitiing on himself.

He'd been planning on doing it for some days, but as he sat in the afternoon sun on the pea-straw bale, waiting in the Shrieking Shack for Amelie, the perfect moment had presented itself. So with quill and ink, he carefully drew the outline of a moth with his right hand into the exact same place his father had his Dark Mark. With nerve-jangling methodical care, he coloured the moth's wings and body, and had just laid his newly inked wrist out before him, drying, when he heard Amelie's feet on the porch and then pushing open the door. Clearly she'd flown again, her preferred means of going home after school.

"Servius?" she called.

"In here, by the window."

She came through, still in Hogwarts uniform and rucksack over her shoulders, her expression apprehensive. Servius thought her pretty, and smiled to reassure her, then saw why she'd been worried. Behind her, eyes wide and amazed as he gazed about him, came Wait for William.

"I brought William," she pointed out.

Servius got cautiously to his feet, and at first the two boys simply stood, regarding each other. Then Servius, who'd wondered if he'd ever get the opportunity to say goodbye to William, felt the relief rush through him. But what he said was: "Sup?"

"Yeah. Nuthin."

"You just checkin' me out?"

"I dunno man," said William, looking aggrieved. "Amelie told me to come. And I was like, okay, safe, whatever. That's cool. I mean, are you coming back to school or are you gone mate?"

Servius lifted one shoulder then, after a silent pause, stuck out his left wrist. "Sick, eh?'

"That is well cool," said William nodding exaggeratedly, and fist-bumped Servius. So it was that in the abbreviated world of boyhood injustices, issues had now been aired, matters had been sorted and all agreed they could be best friends again. "But is that a butterfly, Sev? Isn't that for girls?"

"It's not a butterfly! It's a moth. Like my mum. It's like when you get a tattoo for your mum, cause you love her." Servius considered it thoughtfully a moment, then added, "And it's my mark."

"Can I have one?" said William hopefully, and Amelie rolled her eyes.

As they sat in a circle on the floor, Servius carefully scrawling away on William's left inner wrist, Amelie answered Servius' questions.

"The Imperious curse means that sometimes it was the real Professor Longbottom but he was being controlled by the Rabastan man, and sometimes it was the Rabastan man who had taken Polyjuice potion and was disguised as Professor Longbottom."

Servius shook his head in disbelief. "Can you imagine what Muggles would do if they could have this kind of magic? So which one was wearing the mask?"

"The Polyjuice only lasts for a couple of hours, so I would say that it was the Rabastan man who had run out of Polyjuice or it had worn off or hadn't started yet."

"So now they've got the real Professor Longbottom, and they will crack the curse he's under, what happens to the Rabastan man?" asked William.

"I think they will try to catch him and send him to that prison called Azkaban."

"You think he'll just give up trying to get the Resurrection Stone?"

"He would have to try and get to the tomb without anyone seeing him because he can't pretend to be Professor Longbottom anymore."

"We don't even know why he wanted it unless he thought he could sell it for a lot of money," said Servius. He had finished William's black moth, who now sat very still, cross-legged, holding his left wrist while it dried, and Servius wriggle-turned to start on Amelie's. Her wrist felt incredibly delicate to Servius and his fingers trembled a little as he drew.

"Maybe he was trying to save someone's soul," said Amelie, quietly. "That's what the Centaurs will be doing tonight."

Servius glanced up quickly. "Huh?"

William was nodding emphatically. "It's true. We learnt about it in Astronomy. Tonight's a Black Moon."

"I saw the homework assignment…" said Servius. "What does it mean?"

"On a Black Moon," began Amelie, soberly holding their eyes as though she were telling a campfire story, "the Centaurs use the Resurrection Stone to release one lost soul that's trapped in limbo, a dead being who can't rest, who has unfinished business. It is the guardian duty of the Centaurs, bestowed on them by the Greek god Psyche. Their tomb isn't just a burial place for Centaurs, it is a portal to the underworld, and sometimes Psyche tells them who's to be released, or sometimes they grant the wish of someone still living. But either Rabastan doesn't know this, or thinks if he takes the Stone for himself, he can control it."

Both William and Servius started at Amelie, who suddenly coloured. "What?"

"How do you know that?"

She shrugged. "It's a witch thing."

William glanced at Servius. "I believe her. I saw some of it in a book. Your dad saw it too."

"Tonight is a Black Moon?"

"Didn't you do your Astronomy Assignment?" asked Amelie acerbically. "We did a Lunar Calendar, like the one I saw in Longbottom's room. That's what I think he is waiting for. Tonight."

Servius was thinking hard on what Amelie had just said: sometimes they grant the wish of someone still living. What would happen, though, if Rabastan got there first? What if his wish was granted? Who did he want resurrected – the Dark Lord guy?

I've got to warn the Centaurs!

But he didn't say it out loud, he bent over the drawing of a moth on Amelie's almost translucent skin, her veins a vivid blue, and his mind flashed on what would happen if he kissed it. Then he forced himself to concentrate, blinking to make the image in his mind go away.

"I am worried though," said Amelie, her voice lowering and becoming confidential, "that I have made a mistake."

Servius and William both gave her their surprised full attention.

"The bond, Servius," she said, with a quick glance at William who looked confused. "Did you see what happened to the circle when your father fell down, here?"

Servius shook his head; his memory too scattered and shredded.

"The circle – it glowed. It recognised your father's blood."

"What in the actual fuck are you talking about?" said William.

"Recognised it?" said Servius. "How?"

"Has your father been in this shack before? Could that be your father's blood on the floor?"

William and Servius stared at the stain on the floor where she was pointing. It was darker now, the blood fresher.

"I don't know! Why would you think that?"

"It glowed! Didn't you see it? I think your father is…I think he is caught in our bond."

"What?"

"It may be why he didn't die." Her expression was exactly halfway between distressed and optimistic.

Servius was speechless. "Holy shit…he's like…part of the Warlock blood-bond…?"

She nodded, but looked unhappy. "I don't mind that but…there was someone else in the circle…"

"The arrow?"

She nodded miserably. "The blood on the arrow. I thought it would give us protection but -,"

"Whoever's blood was on the arrow is part of the blood bond?"

The nod had reduced to a single inclination of her head.

"It could have been the real Longbottom that got hit with the arrow!" said Servius immediately, trying to be positive. "Maybe that's why he's still alive now!"

"But why would the real Professor Longbottom go to the Centaur's tomb?" asked Amelie, almost in a whisper. "The Rabastan man would have gone himself…"

"What does that mean?" asked William, wide-eyed. "What does it mean if the Rabastan guy is caught up in this Warlock blood-bond thing?"

"He can't die," answered Amelie. "Not while we live."

Later, in his upstairs room at Slughorn's house, Servius looked out the window, over the roofs of Hogsmeade, to the horizon. The sun, a fierce, flaming white ball in the sky, burnt into his retinas. It was sinking. "Mr Credge will be coming on Monday," Slughorn had said conversationally, only minutes earlier at his dining table downstairs, placing down a paper bag he'd brought from the Hogwarts kitchen. "There's something for you to eat for dinner. I have to see to the Slytherins tonight and the school is in lock down so I might not be back till late. We sometimes get a lot of homesick children on a Black Moon."

"How do you know it's a Black Moon?" asked Servius, watching closely. "Why's there a lockdown?"

"Oh! Nothing to concern yourself with, just stay in the house tonight. Anyway! We should all know in a couple of days what will be happening. Keep your fingers crossed, Servius, it might be good news, you never know. Now stay inside."

But Servius had no appetite for food. He'd gone back to his room, stared out at the sinking sun, the moth on his wrist starting to smart but he barely noticed. If only he had his wand, his broom, even his trainers…Well, if all he had was his wits, then he'd better get a move on.

In a swirl of robe, Snape strode into the Great Hall to find McGonagall up on the dais, her pointed hat on, already standing at her flying owl lectern. She was speaking with grave ceremony. Filling the Hall from side to side were the attentive students of Hogwarts, the long tables set away so they could stand in assembly. At intervals along the walls like sentinels were the teachers, worry etched on their faces, and Snape was assaulted with flashbacks to those last days as Headmaster, his dazed, hollow, robot-like moment-to-moment existence, the throb of his stomach ulcer, the dull thud of his sleep-deprived brain trying to keep up.

"We are confident that this does not pose a threat to students," said McGonagall in a steady voice, "and we have notified all your parents -,"

Sinistra was standing nearby and caught his eye and Snape took up position next to her. The Hogwarts House ghosts floated above, cruising and watching; the Black Moon was significant and energized them.

"…but for your safety, we ask you to stay together tonight in your dorms and the Seniors and Prefects will be keeping watch. Do not under any circumstances leave the castle. I repeat, do not leave the castle…."

"I was going to hold the Stargazers tonight," whispered Sinistra, looking at him uneasily. "It's been cancelled. What's going on? Where's Neville? I can't keep up!"

Some ashen-faced students turned towards them and Sinistra lowered her head. "Is Servius alright?" she asked under her breath. "I'm so worried about him."

"He's fine," murmured Snape.

And at that exact moment, a Patronus burst into the Hall, a Patronus of a magnificent antlered stag and it galloped up to Snape, a hundred eyes following it and McGonagall paused in her speech.

Recognising it as Potter's, Snape hastily exited the Hall and stood in a discreet spot before inviting the stag to deliver its message "Keep Servius safe," came the voice of Potter. "He's appeared in the Foe-glass."

"What does that mean?" said a voice behind him, and Snape swung around to find Sinistra had followed. She was a picture of worry. "A Foe-glass? Why has Servius appeared in a Foe-glass?"

"Rabastan had one in his room," Snape hurried to explain, keeping her close as he spoke. "Potter took it with him as case evidence. If Servius has appeared in it, it means Rabastan views him as an enemy. The Foe-Glass is meant to alert the owner if his or her enemies are close."

"But – Servius should be in Horace's house."

"Indeed…"

Snape reentered the Hall, followed by Sinistra, and discreetly edged along the wall until he reached Slughorn. Up at the lectern, McGonagall was starting to wind up her speech.

"Horace," said Snape in low tones, "where's the last place you saw Servius?"

"At – at home, less than an hour ago. Why?" replied Slughorn, looking rather startled.

"Did you tell him to stay inside?"

"Most certainly! I mentioned there was a Black Moon and a lockdown and that he was to stay in the house."

"Does he know what a Black Moon is?"

"He would have received homework from me in his Dossier," said Sinistra. "We discussed it in Astronomy and he would have been invited by default to the Stargazers Club meeting tonight. So yes, he may well know what a Black Moon is. But whether he's connected it to being a threat I don't know…"

There was a sudden clamour of movement as the students in the Hall were dismissed to go to their dorms, and the Heads of Houses began ushering their members into lines, McGonagall once more taking the Gryffindors beneath her wings. The students gossiped among themselves excitedly, sharing their stories of odd encounters with Professor Longbottom and what possibly could have happened to him, and whether the castle was imminently going to be overrun with ghosts and, if so, the best way to deal with a nasty one.

Slughorn was momentarily distracted with the task of organizing seventy-six excitable Slytherins into rows, and as Snape stepped aside, he was unexpectedly approached by an anxious Madam Pomfrey.

"Severus, Severus – I've given leave to Diaphne, she was worried sick about Professor Longbottom and asked if she could go to his bedside. I know the pair had a close friendship – was I right to do so?"

"But the Longbottom she knew isn't the one in St Mungos," said Snape, but this just seemed to make Pomfrey more confused. "It's fine. She'll discover soon enough."

"Shall I go and check on Servius?" suggested Sinistra.

A harried McGonagall came up to him at that point. "Severus – I'm attending to the Gryffindors. When are Potter's Aurors due? Flitwick has put up emergency barricades and locked the gates, so they'll need to be admitted. You have the charm-codes?"

"Ma'am. I believe they were going to use the cover of night, however I haven't heard from Potter about that. When did Flitwick put up the barricade?"

"About an hour ago when lockdown was enforced. We had a Quidditch team returning from Hogsmeade and now we've accounted for everyone."

"Rabastan may well already be somewhere on the grounds. It's Sirius Black all over again," said Snape running a hand over his face.

"If the barricades are up, you'll need to let me back in after I've checked on Servius," said Sinistra.

"No, Aurora, I don't want you out there alone, it isn't safe. Rabastan knows all the places to hide and is proficient in dark magic. Go to the Slytherin dorm with Horace…I'll meet you there."

"Professor Snape?" he turned then to find Wait for William and Amelie standing behind him, having clearly lagged behind the other Slytherins and ducked away to speak to him. Amelie glanced past Snape to Slughorn, who having shepherded the other Slytherins into the Entrance Hall was now impatiently walking back towards them to collect the two strays.

"Miss Hellmann?"

"Servius is in trouble," she said, her eyes enormous behind her glasses. "I'm sorry!"

"What?"

"I can feel it. He's in trouble. Something's wrong."

"You can feel it…" repeated Snape, remembering the circle in the Shrieking Shack. "Where is he?'

"I don't know…" said Amelie woefully, and lifted her hand to wipe an eye behind her glasses. Snape noticed the smudge of ink on her wrist that resembled a moth and frowned, and she shoved her hand back down to her side quickly.

"Servius is at home," said Slughorn sharply. "I told you. He's in my house, sitting tight like I told him to. He wouldn't go anywhere – his broom and wand are confiscated."

"He doesn't have his wand!?" said William. "Oh no!"

"Rabastan?" said Sinistra fearfully.

Snape's insides turned to ice. "Did you put an alarm on the house?" he asked Slughorn.

"An intruder alarm, yes. I would know all about it if anyone has gone into the house after him."

"But not if he's left -,"

"I told him about the Centaur's tomb," said Amelie. "He knows that Mister Rabastan may come tonight for the Stone. Wait for William and I think he may have gone into the Forest to go to the tomb."

"The Foe-glass said Servius was near," said Sinistra. "If Rabastan hasn't gone to him then -,"

"Would he remember the way?" asked Snape, his feet beginning to move, panic rising like an electrical circuit up his spine.

"He would if he had his wand," said Amelie. "Otherwise I don't know," she gulped back tears and Snape saw Benedict Hellmann crossing the floor determinedly towards them. "I thought he had his wand!"

Snape didn't wait any longer. His son was in trouble, unarmed in the Forest. His feet seemed to move of their own accord as he headed across the Entrance Hall, the others trailing behind him.

"Severus!" cried Sinistra, "where are you going? What are you doing? It's dangerous!"

Snape waved his wand at the bolts and barricades securing the front access, and they seemed to grind their disapproval as they unlocked, allowing the great oak doors to open. Snape curled his fingers more tightly around his wand.

"I'm going to find him," he said.

Slughorn's hiking boots, with several pairs of socks and tied with wrap-around laces, were hopeless for running in, but they did keep Servius' feet warm and dry. Devoid of his wand, for which he'd turned Sluggy's house upside down in efforts to find, he was grateful for at least this small comfort. Atop his fleeciest trackpants and Slytherin jumper, he wore his windcheater, and over the top of that, his Warlock cloak with the hood up to help conceal his face. Snow was not far away, the black clouds could be seen advancing over the mountains far off to the north, but for now the sky was clear and the air chilly.

From the tunnel at the Whomping Willow, he'd run directly down the hill to the Forest. He scarcely stopped to look, but all around the school grounds seemed still and hushed, holding its breath, the lake mercurial and the Forest itself was sullen and inscrutable. He didn't stop to think about it, he couldn't afford to. To dwell on his actions now was to stumble and fall. There was no going back. And if on Monday he was expelled, then everything came down to this next hour, he had no time to luxuriate on rights and wrongs.

He raced down the edge of the forest until he was opposite the owlery, and then whistled piercingly for Tāne, searching the sky…c'mon, c'mon…and then, like a small brown eidolon, his owl appeared from a spot somewhere above the woods and swiftly came to land on Servius' wrist. "You clever thing," murmured Servius, with a numb-fingered tickle, unable to keep down a fond grin even now. "Tāne, listen, I need your help. Guide me through the Forest – I'll follow you. Take me to the Centaurs. Can you do that? Can you take me to the Centaurs?"

Tāne bobbed his head, his eyes bright in the centre of a rim of fire-yellow feathers and then silently launched into the air and flew through the dense shield of regimental trees in their uniform of soot-coloured bark and skeletal branches. Did they lean towards the owl, seem to bend in over him as his wingtips brushed them, use their spindly twigs and leafless boughs to mark or catch him? Servius couldn't be sure, he no longer trusted the impossible or implausible to his own imagination anymore, but he thought maybe they did, maybe the trees were watching, and he swallowed his fear down hard and tugged his cloak around him as he took off after Tāne, as fast as his man-sized boots would allow.

The sun still had some dappled allowance within the Forest and Servius worked fast, soon panting hard as he fought through the undergrowth at feverish speed. The glimpses of his owl were only brief, and the flare of an occasional blue Assingo mark was like a small, cheerful avowal that his route was literally on track. There were some things he was starting to recognize: certain trees and bushes, the location of a particularly nasty stinging plant, that swaying vine that always slung down as he passed beneath it. A big pile of dung was new, however, having the slightly greenish, fibrous feel of a herbivore's doings, and he paused with a heavy sigh to pull his boot free.

He wouldn't have his wand to light the way this time and nerves fluttered in his chest as he glanced upwards, trying to see the sun, but the tops of the trees were too tall for the horizon. Every time he thought of a troll or an acromantula or some other fiend in the forest that could end it all in seconds, defenseless as he was, he replaced it with the image of his mother before the Death Eaters; his father lying to the Dark Lord, watching others get killed, the tortures he endured before staggering to his feet, just to lie again; the scars that laced his body, his heart, his mind.

His father did not run away. This was the Snape way.

The sound of Centaurs Praising reached his ears before he saw them. It was a low, regulated humming sound, not musical but very persistent and seemed to travel through objects as well as around them. When he at last crested the small hill on the edge of the meadow, he beheld the Centaurs all gathered and realized that the clearing had been made by them, using the trees – all oak - at the circumference to shape an amphitheater that intensified the chanting. In the background, monolithic, shadowed and imposing, its arched entrance an impenetrable black, was the tomb.

Many of the smaller Centaurs, including the females, elderly and youngsters, were lying with their hooves tucked beneath them, while around the fringe of the clearing, standing, were the armed stallions. One larger than the rest, at the forefront and facing the group, was a grey stallion with lavender coloured hair, seemed to be leading the Praising using hand gestures like an orchestral conductor, instructing when to change pitch.

Servius decided he was the right Centaur to approach, and stealthily began to make his way towards him using the oaks around the perimeter as cover. Above him Tāne lightly flitted from tree to tree, but when Servius reached the edge of the clearing level with the grey Centaur, the owl melted into the backdrop of rough-barked oaks, looking almost identical to a little stump.

For some minutes Servius hovered unnoticed, but the Praising was continual and showed no signs of stopping. Eventually and with a hammering heart, he took a deep breath and almost mentally pushed himself forward out of the trees, his empty hands held high above his head and his eyes tightly shut.

Almost immediately there were shouts, and then the humming dropped away. Then there were cries of alarm, accompanied by the sound of thudding hooves and Servius opened one eye. Coming straight for him, bow drawn and arrow pointed, was the grey Centaur.