Disclaimer: seriously, these people and places don't belong to me (except for the occasional potions geek). Hogwarts and its people belong to JK Rowling and Warner.

A/N: Thanks to those who reviewed – I'm amazed at your stamina for keeping up with this for so long. Cyber-peppermints to you guys.

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Chapter 70: The Common Denominator

Dumbledore pressed his hands together. "It all seems to have been an accident," he said, then, as Draco shifted in his chair, exchanging a look with Harry, the ancient wizard sighed and added, "And, of course, like most accidents, one that could have been avoided with a little forethought." He sighed. "Remus? Do you have anything to add?"

Lupin shrugged; grimaced. "Mea culpa? I thought things were under control, but obviously I overestimated the power of Luna's wall of sky… and underestimated the power of Elmsworthy's new potions. I plead guilty to poor judgement."

"I don't understand why you could underestimate those explosions," Draco complained. "I mean, the dungeons are still drying out, after all…"

Remus looked down at his folded hands. "Yes. However, there is a certain significant variable at work, which… Headmaster, we really need to discuss this immediately. And with Miss Lovegood."

Dumbledore's brow creased so quickly Harry nearly missed it. He took out a pocket watch and flicked it open. "It would appear we are in luck – she's on her way now."

"Er… is this about… that thing?" Sirius said.

Draco and Harry exchanged another look. "Is this a thing we aren't meant to know about?" Harry said carefully.

"It is a thing you are about to be given total and totally confidential information on," Dumbledore said, sharply enough that Flitwick looked up from his examination of Sirius' transcribed statement with a raised eyebrow. "And on this, there will be no excuses for revealing the secret, no accidental slips of the tongue. Am I clear?"

"Yes, sir," said Harry, quietly worried now.

Draco nodded, too. He said nothing, but he seemed more than usually on edge.

Dumbledore met his grey eyes. "It does coincide with the particular item you mentioned to Professor Lupin and myself this afternoon, Draco," said Dumbledore more softly. "Although I haven't told anyone else about it. I would quite like to tell Professors McGonagall and Flitwick."

Harry felt a sudden weight he hadn't known he'd been carrying lift and evaporate above his head. Flitwick suddenly became extra interested in something on the scroll, although Harry had the impression his ears were almost out on stalks.

"Yeah. Okay." Draco pursed his lips. "I told Professor Dumbledore when I had study period. Lupin as well," he admitted grudgingly to Harry, who must have been showing his astonishment at the fact that Draco had confided anything in Dumbledore, especially something so potentially damaging for his mother. It was even more mind-blowing that Draco had also included Lupin.

Dumbledore was smart enough not to push for a 'Professor'. But even so, Draco was looking increasingly uncomfortable. This latest fiasco with Simon must be making him wonder if he'd been insane to cast his lot in with this pack of incompetents. Harry hurriedly racked his brain for an appropriate next topic before Draco lost his nerve…

"Er… it's about Simon, isn't it?" Harry said. "Why we're waiting for Luna, that is."

Remus and Sirius looked at each other. Dumbledore twitched in surprise. He readjusted his glasses. "Dear me, what made you think that, Harry?" he said softly.

Draco, flicking a glance at Harry, answered. "Potter and I are here. You want to talk to Luna as well. The common denominator is our horse."

"It's best if we – ah, here she is now." Dumbledore beamed as Professor McGonagall opened the door and led in Luna, who blushed all over again and refused to meet anyone's eyes, especially Harry's.

"Hey, Luna; we're in trouble," said Sirius, trying to lighten the atmosphere. "Think we'll get detention together?"

Luna managed a weak smile.

"Miss Lovegood already has it," said McGonagall. She smiled coldly. "You know, I don't think it's too late to assign detention to you as well, Mr Black."

"I was joking…"

"I wasn't," she replied.

Sirius looked down at the carpet, squashed. Harry couldn't help feeling sorry for him, even if Sirius had been a colossal prat.

"And as for you, Prrrofessor Lupin…

"Remus has something to say about the flux," Dumbledore said, interrupting McGonagall before she could get a good head of steam up. "It's important that Luna be consulted on this – it concerns Simon's welfare."

"Simon? Oh, the horse. I don't see how… Oh. Oh. Yes. The horse." She glanced at Draco. "So you've worked out the parameters, then?" McGonagall enquired of the werewolf.

Remus looked grim. "Three months."

"What?"

"Three months. That's all we've got before the magic is leeched out of the stones to the degree where regeneration collapses. That's what Voldemort is waiting for, I suspect. At that point the wards protecting the school from attack will be so weak the Death Eaters will just walk through them."

Flitwick squeaked in alarm and dropped the scroll. McGonagall went pale. She sat down.

"Elmsworthy's explosion down in the dungeons, while quite violent, should never have breached the walls," Remus continued. "I factored that into my calculations and, well, got a rather nasty shock."

McGonagall nodded. She looked like she'd had a rather nasty shock, too. Dumbledore poured her a cup of tea and refilled Flitwick's. McGonagall picked up her saucer and the cup rattled briefly before she stilled it with the other hand.

"How does this concern Simon?" Luna asked carefully.

"We need Simon to carry two people through the barrier," Dumbledore explained. "Early on Thursday morning, to be precise, in order to attack Voldemort and bring down the barrier."

Luna went white. It was doubtful she was shocked by someone saying 'Voldemort'. "You can't," she whispered. "He – Simon's not able to do that. I need more time… to, um… to train him…"

"We don't have time," Dumbledore said softly, his face grave. "I'm sorry. But this is a desperate situation and we need to take desperate actions."

Harry thought that sounded a bit dramatic. Luna was shaking her head. "No, no… it's too dangerous for him… I swear, Headmaster, I swear I'll be ready soon, but…"

Dumbledore sighed tiredly. "We both know that isn't going to happen."

Luna bowed her head and covered her face with her hands. A long, shaky indrawn breath later, and she lifted her head again. Her eyes were dry and bleak. "So you've given up."

Dumbledore flinched as if stung. "Never," he said firmly. "I stand by what I told you at dinner last evening. But you know as well as anyone that if Simon understood the situation he would volunteer to go through the barrier despite the dangers."

Luna glared at him for a moment, and Harry was shocked to see something like hate in her eyes. Then that anger or hate or whatever it was crumbled and left her silvery eyes desolate. "I know. But why does it have to be him?"

Dumbledore stared at her silently, his eyes compassionate.

"I don't have any choice in this really, do I?" she said at last. "No, I thought not," she added bitterly. "So. Who's going to ride him? Don't tell me you're going to throw Harry to the mercy of the Death Eaters."

"Of course not. I am the one who will be going," said Sirius.

Luna laughed, slightly shrilly. "Oh," she said, sobering almost immediately. "I thought that was a rather good joke. Like the one about a horse going into a bar." She peered owlishly at Lupin. "I suppose next you'll say you're the other one he'll be carrying?"

"No, I've lost my capacity for blatant optimism. And he won't carry me. Or Sirius."

"Yes he will bloody well so carry me," Sirius snapped at Lupin. "If you think we can stand to lose –"

Harry frowned and opened his mouth to ask the obvious question. Draco beat him to it.

"What are you on about? Simon carrying people through the barrier? I suppose it's escaped your notices that he's not really all that fond of carting around people he doesn't know, let alone through the barrier. It took me a he- er, a heck of a lot to convince him to go through last time. You can't seriously expect him to drag any of you through it unless you put him under the Imperius…" Draco trailed off. A sudden ugly, angry expression twisted his mouth and pink began to stain his cheeks.

"There will be no spells cast on Simon. Especially any of the Unforgivables. Yet he will carry me," Dumbledore said serenely. "I am hoping he will carry Professor Flitwick as well. Simon seems quite fond of you, Filius."

Flitwick nodded reluctantly at this dubious evidence of favour in a horse's eyes. "If it means putting an end to this, then yes, I will accompany you. But, Albus, you–"

Sirius exploded. "Absolutely not. With all due respect, Headmaster, you're off your rocker if you think Hogwarts can afford to risk losing you. Er, you too, Filius," he added hastily as the Charms master frowned. "I'll go. Hogwarts can afford to lose me, and you know I'm damned good with a wand. Almost as good at duelling as Filius, in fact."

"Yes, you're good, but with a wand which you don't have," Dumbledore pointed out. "The one you have been borrowing – I've pretended not to know where you acquired it, but as soon as Auror Price is returned to his right mind, I trust his wand will be suddenly available to him again – the wand you have is not keyed to you as a properly selected wand would be. I've considered the possibilities of people able to ride the horse through the barrier, but I'm afraid it's limited to myself, Filius and Madam Hooch. And Rolanda, while capable, is not of a calibre which can stand up to Lord Voldemort. Only I can defeat Voldemort, although I expect Filius could give him a good run for his money. I'm hoping if I am unsuccessful, Filius, I will have weakened him enough for you to finish him off."

Flitwick nodded, although beneath his mop of fluffy white hair there were lines deepening around his eyes.

"So," continued Dumbledore, "the list of riders is a limited one."

"You left out me, Draco and Luna," Harry pointed out quietly.

"You're indispensable, Harry," Sirius pointed out. "The prophecy –"

"Says that I have to face Voldemort and defeat him. Or he will defeat me. I found that out in the Department of Mysteries last year, remember?" he added bitterly. The memory of how easily he'd been tricked still rankled. Not to mention the burningly cold horror when he'd thought Sirius was killed by Bellatrix LeStrange. No matter how much power Sirius had to piss him off, Harry never wanted to feel that terror again. "I'm the one who has to go, if anyone."

"Don't place all your faith in prophecies," said Dumbledore. "Besides, Harry, I'm afraid it's time we stopped treating you as our weapon and started treating you as a person. You are still a student."

Harry breathed heavily through his nose. "Forgive the impertinence, sir, but it's a rotten time to suddenly treat me like a child," he said, spitting out the words. His hands were shaking. He realised, on some displaced level, that he was angry. "I can go. In fact I'm the best choice…"

"I forbid you to go," said Sirius heavily. "You are too young. Dumbledore, I may not be your godfather, but I suggest very strongly you take my advice. With some more riding lessons, I –"

Before Harry, fighting to see through a rising red mist, could find words, Dumbledore intervened. "There is not enough time for the riding lessons you would need. Minerva, Filius, Miss Lovegood… I am afraid we are running out of time. And there are hints that Voldemort is key to the integrity of the barrier…"

Harry hadn't know that. A new chill that cut through the rage. It seemed like every time they found a way forward, Voldemort was there, blocking their path. On the heels of his fading fury a wave of helplessness rolled over him. He rubbed his face with his hands, feeling the scrape of stubble which told him he was getting near to needing a shave again. So close to being a man – but would it give him the strength he needed? All he ever seemed to find on the brink of adulthood was an ever-increasing depth of confusion. The powerlessness of childhood was being exchanged for this new, murky dimension where good people like his father and Sirius showed themselves up as corrupt, and evil was a slippery fish that slid through his fingers every time he thought he had a grip on it leaving him with nothing but poisonous slime.

Did Ron and Hermione ever feel this way?

He looked sideways. Malfoy's face was a perfect blank now, but his grey eyes glittered in the tail end of the day.

Harry wondered, not for the first, second or even twentieth time, what it would be like growing up as a Malfoy – growing up under (from the little Harry understood of French) the name of Bad Faith. Oddly, this made him feel better about his own lot. Sirius and Dumbledore might be trying to run Harry's life for him at present, but it didn't mean Harry would let them. They wouldn't try to twist his life to their own purposes in the same way or with the same intensity Lucius Malfoy would Draco's. And with that thought, the terrible feeling of helplessness fell away, although it left a twinge of sympathy for his Slytherin… ally.

Dumbledore was still speaking. "…Remus, you are sure of your data?"

"Yes, Headmaster. The fields of the castle walls are definitely showing the first signs of decay. It would have been going on for some time, probably even before the barrier itself was solidified, but the decay is exponential. It will only get faster. As for the barrier, the monitoring charms show that the causative locus moves independently of the three anchor trees. I expect the causative locus is our favourite neighbourhood Dark Lord. I don't think You-Know-Who remains vital to the stability of the barrier now that it's been completed, but…"

Dumbledore lifted his glasses and pressed his fingertips against his eyes. "Yes. Although we can probably lower the barrier without needing to resort to killing the causative locus, for reasons of greater security it is imperative we find and destroy Voldemort. What is vital is the knowledge of where he can be found. Mr Malfoy, would you like to tell those who do not already know what you found out today from a source which I believe is reliable." He lowered his hand and somehow managed to pin everyone in the room with a sober gaze. "I must stress to everyone that this source is to remain totally confidential and must not even be speculated upon in private. Draco?"

Draco nodded reluctantly. "There is to be an attack on Hogsmeade. This Thursday. Four, four-thirty in the morning. Oh, and I don't consider myself dispensable, thank you, Black. Probably not even Luna is dispensable, although after that piece of idiocy you and she got up to that's stretching probability."

Luna and Sirius had the sense not to argue that. No-one wanted Draco to walk out at this point.

"So you intend to ambush him, Albus?" asked McGonagall, who was slowly regaining some colour.

"Yes. He won't be expecting me. And this time I won't let him escape…" Dumbledore steepled his fingers before him, blue eyes staring into some bleak realm open only to him. "I cannot afford to let him escape again."

"You can't afford to go," Sirius argued. "If Hogwarts begins to crumble it'll need the headmaster to stabilise it."

"And if I die that role immediately goes to Minerva. She will inherit the spells to maintain the castle," Dumbledore said matter-of-factly, ignoring McGonagall's muffled mew of protest.

Draco sat back, folding his arms, a mutinous gleam in his eyes. Harry caught his eye and shook his head and Draco caught the hint and held his tongue.

"Is this meant to be some sort of cry for help, Dumbledore?" Sirius snapped. "Because it's bloo-"

Dumbledore held us his hand. "Sirius," he said wearily. "Do you think I would leave Hogwarts if there was an option? Voldemort needs to be defeated. I am able to do that, but only if I know exactly where to find him. Tom, slippery fish that he is," (Harry twitched at having Dumbledore echo his earlier thought) "knows this and has done his utmost to elude me since his return to power. Even in the battle the other month he kept his Death Eaters' eyes on me, using them to see where I was so that he could be somewhere else. This is a sterling chance, the break we have hoped for, and one that is unlikely to come again. Enough arguing, please – everyone. This is the best option and I am sure upon reflection you will come to agree with me. Now. Draco. Was there something else you wished us to know?"

Draco's mouth pursed mulishly for a moment. "There is a potion. But I don't want it traced back to me. Which it would be easy to do if any – and I mean any details about it were known."

"You have my word," said Dumbledore. "And the rest of you?" he said to the room in general. There was a chorus of ayes from Flitwick, Sirius, McGonagall, Luna and Harry.

Draco nodded. "Hmm. The potion is designed to weaken the Dark Lord. I don't know to what extent – it's hardly as if it's ever been tested before, of course."

"There's a certain lack of Dark Lords running around willing to offer themselves as test subjects," said Sirius, returned to trying to lighten the mood, although his face retained a tinge of its angry colour.

Even Draco managed a small smile at that. "That's the trouble with research – finding test subjects."

"Being blown up is somewhat off-putting to the more thoughtful brand of volunteer," said Remus, sticking his little finger in his ear and wriggling it. "I really need to see Poppy to check if this deafness is going to be permanent…" he grumbled. "I hope you're not going to involve Elmsworthy."

"I'd rather not show the potion formula to anyone who isn't directly involved with its brewing or application," said Draco anxiously. For the first time he looked like he could bolt.

Dumbledore nodded. "That is perfectly understandable. I give you my word on that one, Draco. Filius?" he added, as Draco visibly relaxed.

Flitwick's nose twitched. "My goodness. I've never been much of a brewer. Can you still remember the lessons you had from Flamel?"

"Oh, I'm not quite so decrepit I've forgotten everything he taught me… I even filled in for Professor Boggle on a few occasions."

"And you never heard complaints from the students over it, either," said Lupin stoutly. Sirius nodded.

"Thank you, Remus. Some of his ailments were very strange. We never did work out how he came down with galloping tsetse for an entire week in April…"

"Probably safe enough at this date to say we were pretty sure it was Severus who was responsible for that one," said Remus. "The students all decided to voluntarily develop amnesia over the source of Boggle's affliction."

"Really?" Dumbledore raised an eyebrow at Sirius. "I'm surprised that you never informed anyone, if you knew about it."

Sirius shrugged. "Well, I hated Boggle a little bit more than I hated Snape. Most days, anyway. I was hardly going to stick out my neck for the old bast- er, Boggle. Also he'd just given me a detention which I ended up not serving. I swear Snape went green when he found out he'd done me a favour." He smiled.

"So are you willing to show the recipe to me?" Dumbledore asked Draco.

Draco nodded reluctantly. "Harry and Luna know about it, too. Although they don't know too many details. Oh, and Granger. Weasley lent me his owl, but he doesn't know the…"

There was a cough from McGonagall. "…Sorry," she gasped. "Tea went down the wrong way. I could have sworn you said Ron Weasley lent you his owl…"

"Pigwidgeon. Yes. Granger's cat was useful, too." Draco stared at her a moment longer, perhaps to see if she would have a heart attack over the shock of him and Ron working together. "So Weasley knows I got a message. Elmsworthy's probably guessed, but although it might be useful to get him to help with the brewing," he added with glum resignation, "I really, really, do not want him to know where the formula came from. There must be some way of tricking him into thinking the formula and the message are unrelated, although we might be able to do without him. Granger… I don't think she'd blackmail me at a later date. Weasley might," he added thoughtfully, pulling at his lower lip.

"He won't," said Harry.

"Hn."

Which could have meant anything.

ooOOoo

They agreed to meet up later, down in Snape's workroom, going separate ways at separate times so as to minimise suspicion. Dumbledore gave them the passwords – small silver keys which shimmered and then evaporated when they put them on their tongues. Harry accompanied Draco down the stairs towards the Entrance Hall, discussing in low voices the differentials of temperature needed to prime the base for the potion they would be brewing later that evening. After going over the formula with Harry, Draco and Remus, Dumbledore had considered the preliminary base simple enough for two students who'd effectively completed sixth year Potions, although he would be there later for the trickier aspects of the bottling spells. "Need me to bring anything?" Harry asked quietly, so that not even the portraits could hear. He caught a glimpse of yellow and waved to Stephanie, who nodded back from where she was gathering fallen apples with what looked like a satyr. The satyr took advantage of her moment of distraction to try running his hand up her skirt. Stephanie kicked him in the Pippins.

Draco gave him a wry look. "Patience. A sense of humour. Anything to distract me from – oh bugger."

A small, familiar shape dwarfed by a large camera waved up at them hesitantly. Harry waved back, wondering what Colin was up to now. Draco stopped on the landing. "Damn. I just remembered. I'm meant to be taking that Creevey idiot up to take some pictures of Simon."

"You're joking."

"Did you hear any mention of my horse walking into a pub? No, I thought that as we've got good light late in the evening, he wouldn't need that light thing that scared Simon last time."

"All right, all right… I can chop up ingredients as well as the next person – that being you rather than Hermione or Elmsworthy – and I know the difference between simmer and stew, so I can get everything started okay, but this is a bit of an about-face, don't you think?"

Draco shrugged. "Better he be supervised rather than try sneaking into the paddock and scaring the hell out of Simon. I don't want my horse getting in trouble for defending his territory."

Harry ignored the 'my horse'. It wasn't as if anyone including Draco really believed Simon chose to belong to one person more than another. Or to anyone. Harry strongly suspected the truth of the matter was that the horse regarded them as its own personal property. "I don't think Colin would provoke him – not like Myers and those other two."

"Blume and Hitchcock. Mm."

Draco knew their names. That boded badly for the three who had irked Simon. Harry swallowed a smile.

"Maybe not," Draco continued, "but it's better to be sure. He asked me; don't know why he didn't ask you…"

Harry shrugged. "Guess he felt he had to face you over it." Harry didn't quite understand his reasoning, but perhaps it had something to do with confronting demons. If Colin could get to Simon through Draco, well, that was quite a feat, especially given the last time he'd taken photos.

"Huh. Could be something to that Gryffindor bravery rumour. I thought that if I go up with him I could, I dunno, stop him from doing anything stupid." Draco paused. Frowned. "Anything majorly stupid, that is."

Harry gave a lopsided smile. "Good luck."

"Thanks. To tell the truth, it's just nice to get up to Squirrel Hill to spend time with a horse. Much less complicated than anything in the castle. You coming, or are you going to see what sort of story Dumbledore is going to try feeding the masses this time re. your psychopathic godfather? Tempting to stick around just to hear the story – God knows I could use a laugh."

Harry ignored the insult to Sirius – Draco had cause. And he'd trusted Harry with the formula, now folded in Harry's pocket. "Didn't you want me to start that base for the potion?"

Draco leaned against a wall and frowned down at his shoes. "Might be best, yeah. Luna was going to give it a go, but I guess she'll be too busy with detentions to do it. And we really should get things right as soon as possible – we don't have that much time…" He sighed. "I'll try not to be out too long… nice to have a pass allowing us out until midnight. Hope that means we're allowed to skip the odd class here and there. The odd extra class here and there, 'cos I've been missing quite a few over Simon…"

Harry considered that frown. "You okay with Dumbledore being involved?"

"Yes. No. I… Just don't let him find out anything more, okay? It's bad enough he knows I got a message. And I had to tell him who sent it, too… I don't want him finding out anything about, well, me, for instance. You know what I mean. Make sure Black knows he's not to open his blabby big mouth to anyone who doesn't already know about where the letter came from."

Harry nodded. "Is it okay for Hermione to work on the potion?"

"To be frank, I don't think we have any choice. I looked over that formula and it was, well, pretty nasty. It needs someone who can think their way through a corkscrew – some of it I don't understand at all – what sort of a measurement is 100C when it's not a temperature?" He shook his head in disgust, then looked at Harry hopefully. "Don't suppose Snape mentioned anything like that when you were back in time? 'Cos it's something to do with his calculations."

Harry shook his head. "Sorry."

"Too much to hope for. Huh. Best to have Elmsworthy, but I don't want to tell him where the recipe came from. If all else fails, tell him it, um…"

"How about we tell people Dumbledore found the formula in an ancient scroll that fell out of the Sorting Hat?"

Draco sneered. "Who the hell would believe something like that?"

"When the basilisk attacked me, I pulled a sword out of it."

"Oh. Really? That's pretty cool, actually. All right – pulling the answer out of a hat probably isn't as outlandish as some of the things we're expected to swallow around here. Well, I'll see you in an hour."

"When you come into the room, remember not to touch the door or the door frame," Harry warned. Then, when Draco sniggered, added, "What?"

"Nothing."

But Harry thought he heard Draco mutter a faint Puk-puk-puk-erk under his breath as he walked down the stairs and across the Entrance Hall.

Harry sighed. He'd wondered when Draco would start giving him hell over that whole chicken fiasco.

ooOOoo