He backed Potter to the wall, never letting go of his arm. Potter's eyes were steady, and he didn't flinch. There was more hatred in his face than Severus could ever remember seeing, but this wasn't the red-faced, spluttering anger he'd managed last year in their Occlumency lessons. This was dark, coiled and cold as one of the stone serpents that made up the Dark Lord's throne.

Yes, this is not the boy I knew.

And Severus meant to learn why that was so, now.

"You were not at the Feast, Potter," he whispered. "I know it." His eyes flickered down to the boy's robes, and caught sight of mud on the hem. It had rained most of that day. "You took a walk to Hogsmeade." He leaned in, sending out a knife of Legilimency that would subtly cut apart the unaccountable Occlumency shields in Potter's head, slowly enough that he would not notice until they fell. "And you—"

He paused. An acrid smell drifted to his nostrils, one he would have expected in his classroom or his office but nowhere around Potter, who was not taking Potions this year. At once, his dread deepened, and his anger rushed to the surface, nearly strangling his voice as he drew his wand.

"Accio belladonna."

A cluster of leaves shot out of Potter's robes, smacking into his palm next to his wand. Severus hissed, and was glad that his skin was whole. So deadly was belladonna that even handling the leaves when one had a small cut on the palm could lead to poisoning.

He is creating a poison, or, at best, a draught to make someone sick for days.. Not even Albus can excuse this. His ideal student was plotting a murder.

"I will have the truth from you," he said, returning to eye contact with the boy. His Legilimency knife was digging deeper and deeper, and though it hadn't yet found a chink in the shields, that only increased Severus's opinion that he was dealing with a possessed Potter. The Dark Lord could manage this level of Occlumency without trouble, and his presence in Potter's head would account for everything else, as well, from the changed behavior to the Siren Song that Severus had felt last night.

Potter said nothing. His breaths were quick and hoarse, and he did not look down or back away from Severus, contradictory signals. Severus sneered a bit. Of course the Dark Lord would not suspect that anyone could find him out or check him.

"What do you have to say for yourself?" Severus demanded, and gave the boy such a shake that his head flew back and impacted with the wall. "You have one last chance at explanation before I summon Dumbledore."

Potter exhaled, as if he'd made a decision. And then he twisted and rolled in Severus's grip, so suddenly that his fingers loosened in shock. Instinctively, he grabbed to keep the boy from running.

But Potter hadn't been trying to run. He'd been trying to draw his wand.

"Obliviate!"

Snape's face went slack. Harry took one moment to feel pure relief. He hadn't been sure that his hasty Memory Charm would work.

In the remaining moments before Snape returned to himself, he snatched the belladonna leaves back and stuffed them into his robe pocket, then cast a cleaning spell to rid himself of mud. The Invisibility Cloak went into another pocket, and by then Snape was starting to blink and come back to life.

"I won't come to detention with you tomorrow!" Harry hissed. "I don't want to! A week of detentions with you, and what you made me do on Tuesday, is too much! I'll skip them if I bloody well want to!"

Snape gave his head a shake and focused on him, scowling. Harry wondered if he would notice a blank in his memory, but he was counting on the fact that, even if he did, the git wouldn't want to display any weakness in front of his least favorite student.

And he did not.

"Your attitude needs correcting, Potter," he whispered. "You will come to detention with me all day Saturday."

Harry lowered his head and broke eye contact with Snape. He noticed the small, sharp bit of Legilimency in his head, and rejected it swiftly. He would have to watch that. Though Snape was more skilled as an Occlumens, he obviously had a dangerous subtlety at the mind-reading art.

"I can't, sir," he said tauntingly. "Slytherin and Gryffindor are playing Quidditch on Saturday, remember? Heads of House have to attend the game." He looked up and regarded Snape with a sweet smile.

The ache in the back of his head, and the horrible fear he'd felt clawing at him when he realized Snape might figure out what he was doing, were both worth it to see the way Snape's face twisted and his eyes flashed.

"You will come to my office at eight-o'clock on Saturday morning, Potter," he said. "I do not care how much time you need to train. You, at least, will not be attending the game."

He doesn't know I'm not on the team any longer. That would be a nice surprise for him when he finally inquired. Just now, though, Harry didn't want to bring it to his attention. It might be a bit too much strangeness, convincing Snape he was too unlike himself and that Snape needed to investigate further.

"Always happy to oblige you, Snivellus," he said.

The hand on his arm tightened once more, but the doors of the Great Hall opened then, and the first students began to leave. Snape gave him one more look, full of dark promise, and murmured, "I will see you at seven-o'clock tomorrow night, as well, Potter. And dare you imagine what will be waiting for you?"

He turned and glided away.

Harry stood where he was for a moment, ignoring the curious glances a few people threw him. Let them assume what they wanted. They were from other Houses, casually interested in him if they were interested at all. They would forget all about him in minutes.

For the first time in his life, Harry was grateful that so few people knew him personally. What he planned would have been impossible without that.

A flash of pale hair caught his attention, and Harry moved towards a side corridor, his hand resting on the Cloak in his pocket.

It was time to go Malfoy-hunting.

Severus cursed, cast a preservation charm on the Veritaserum counter-potion to hold it in its current stage, and moved away from the cauldron. His hand was shaking.

It had been years since anything had affected him so adversely, short of actual torture.

He could not remember what had happened between the time he had stood up from the head table in the Great Hall and the moment he met Potter's spiteful defiance outside it. But he could guess. He did have a sensation of contact with another mind, of either his Legilimency or someone else's reaching into darkness and being repelled.

The Dark Lord was in the school somewhere, yes. Either he had reached through Severus's Mark and then cast a Memory Charm to cover his presence, or he had done something similar through Potter.

Severus did not think the Dark Lord had discovered he was a traitor, because of the simple fact that he was still alive.

And that meant caution. Whatever the Dark Lord wanted to find, he could not have found everything. Severus would go to Albus and make his suspicions clear tomorrow, emphasizing the necessity of watching Potter and those few other students in the school who might be candidates for possession. Perhaps the Dark Lord had left some fragment of himself in young Ginny Weasley from her adventure with him in her first year.

And he would take precautions against Memory Charms when he was alone with Potter during their detentions.

He did pause, now, wondering if Potter could have cast the spell himself to protect a secret he held dear. Then he shook his head impatiently. Potter could have the power, the skill, and the determination if the Dark Lord reached through him and wielded him like a puppet, but on his own, the thought of his being able to cast a Memory Charm that could affect Severus was as laughable as that of his being an autodidact in Occlumency. Everything noticeable or unusual about the boy, such as his talent with Parseltongue, came from his enemy.

The true answers, of course, remained locked behind a seamless black wall in the back of Severus's mind.

His spying career might be coming to an end rather sooner than he had envisioned.

Harry, under his Cloak, followed Malfoy calmly into the dungeons, and the time he'd been waiting for finally arrived; the other Slytherins he'd walked with peeled away and left Malfoy alone. The ferret himself paused in front of the blank wall Harry knew held the door to the common room, his face thoughtful.

Now. You won't get a better chance.

Harry began to hum beneath his breath. The Siren Song was much easier this time, maybe because he'd successfully done it once, instead of practicing it without a target or learning in theory. Harry could feel his Occlumency extending outwards like a second limb, aiming at Malfoy, weaving memories of the Slytherin boy into the melody the way he'd used memories of Snape before.

A sigh touched his ears. Then Malfoy turned and walked a few steps in his direction, legs unnaturally stiff, but moving more smoothly when Harry thought that he wanted them to do so. His mind hovered in Harry's grasp, too, a second heart, smaller and slicker than the one in his chest.

I could crush it.

Harry jumped and shivered like a spooked centaur. He didn't like the thought. He had no reason or wish to kill Malfoy. He was only a test subject. Harry knew he would have to perfect the Siren Song before he could use it to call Voldemort, a much stronger Occlumens, but testing it over and over on Snape would be too dangerous.

He didn't want to kill anyone but himself. And Voldemort, but that was the same thing, really.

I need to think about this, and clear my head. And I know just how to do it, too.

He let Malfoy's mind go with a snap, and whirled out of the alcove he'd stood in. Malfoy collapsed to the floor with a groan. Harry decided he must have the same kind of aftereffect that had felled Snape last night.

I'm sorry, he thought. But I'm only going to hurt you, not kill you, and it'll only last a little while. Then I won't be able to hurt anyone anymore.

He knew that the other students left for Christmas holidays on the twenty-first of December. He thought he could be ready a few days after that, given the amount of practice he would have with the Siren Song and Medea's Draught throughout November and the first three weeks of December.

I'm dying on Christmas Eve, then?

I reckon that's it.

A great peace flooded him. Though what he had to do to restore his perspective was anything but peaceful, it felt—right—to know the date of his death. Only that long, and no longer, would he remain in the world to cause suffering, or dodge around Snape, or prolong his payment of the debt he owed Sirius.

Everything will be all right.

Severus felt the Siren Song the moment it began. Though it was not directed at him, a sensitive Occlumens could usually feel it; he had been able to do it every time the Dark Lord used the technique in the past.

He strode swiftly from his office in the direction of the calling, but it stopped moments after it had begun. Severus paused, his heart pounding, and drew his wand. He could not comprehend whom the Dark Lord had been hunting this time, but it would pay for him to approach the area cautiously.

Then he heard a pitiful groan, and recognized the voice.

"Mr. Malfoy?" he asked, stepping around the corner and finding Draco curled into a ball on the ground. Draco whimpered and stared up at him with glazed eyes.

"It hurts, Professor Snape," he whinged, massaging his temple.

Draco was the Siren Song's target. Severus knelt beside him and took his shoulder. "Can you rise on your own?" he asked. Draco shook his head, then blanched so strongly he looked as if he would pass out. Severus helped him gently to his feet, and took him towards his office, where he had headache potions.

"What happened?" Draco whispered.

"A test," Severus said simply. That was his best guess. Perhaps the person the Dark Lord possessed would test everyone in the castle who was supposed to be loyal to him. Of course, he and Draco were the only ones who had Occlumency, so he would probably choose another form of test for the others. Severus cursed under his breath. That meant he could not be sure of catching them.

With another glance to be sure that Draco was fully distracted by his own pain, Severus whispered, "Accio Invisibility Cloak."

Nothing happened. If Potter was the Dark Lord's tool and had been here, he was long gone.

Severus ground his teeth and continued his guidance of Draco. Strange events clustered around him, and he could make only the faintest of guesses how they might be connected. He needed an insight that would undergird all of them, give him the pattern that made sense, but so far he was blind to it.

Harry leaned back in his bed and listened to his mates' breathing. They were all asleep, except possibly Neville, and Neville usually fell asleep again in a few minutes if someone woke him up anyway. Harry didn't think they would sense what he was doing.

For the first time since early August, he lowered the walls that he'd clustered around his scar, around the connection between him and Voldemort.

There was only blankness on the other side. Voldemort still maintained control of his own Occlumency, then.

Harry licked his lips. It took strength and courage, but he had both of them; he couldn't imagine not having them, now that he had only a limited amount of time in which to do everything he wanted. He extended his Occlumency like another limb again, this time trying to slip around the shields and into Voldemort's dreams, or his waking vision.

It felt—slimy. Meeting Snape's Legilimency was uncomfortable, and Malfoy's defenses had been pitiful, but Harry hadn't felt as if he were crawling belly-deeper through a sewer. He grimaced, and envisioned a snake to himself, an exercise that his book recommended. He couldn't just shove forward, not against someone as skilled as Voldemort. He had to slink, be graceful, be quick and wise.

And then there was the shape of a door in front of him, with room under it for him to slip through. Harry couldn't see it, but he could feel it. He imagined easing his way beneath, his imagination and mind compressed into something delicate and unnoticeable. He would just go a short way beyond it, and then—

There was light.

Dim light, Harry saw at once. Voldemort sat on a chair of some kind, and his eyes, through which Harry saw, were fixed on a circle on the floor. The circle was made of some smoking blue liquid that might have been a potion. The steam it shed, though, wasn't thick enough to prevent Harry from making out the wizard who lay in the center of it, chained hand and foot.

"I ask you one more time," Voldemort said, sibilants seeming to glide around every word, even the ones that didn't have an s in them. "Where are the Boltley children?"

The wizard shivered, but didn't answer.

Voldemort gave a slight nod, or so Harry assumed from the way the vision bobbed up and down. "Bella," he said.

A long curtain of black hair shielded Bellatrix Lestrange more than her torn robes did as she stalked forward. Harry felt a wave of hatred, and beat it back. Too many strange emotions would warn Voldemort that someone else was in his mind.

Voldemort did pause for a moment, as if he wondered where the hatred had come from, but then became absorbed in watching his servant again. Bellatrix knelt beside the chained wizard and caressed his face. Where her hand moved, Harry saw, it slit the skin, parting it as easily as the edge of a sharpened knife. The wizard's cheek gaped, and Harry saw red-stained teeth through the mess of his gums.

Bellatrix laughed, and slid down further, and kissed. The wizard screamed, but it was a tormented, gurgling sound, as if he couldn't get enough air into his lungs. When Bellatrix pulled back, Harry saw that his lips were blue-purple, and his hands clawed as if they wanted to rise to his throat. His eyes had gone wide and staring, perhaps mad from the pain of whatever spell his torturer had used.

"Continue," Voldemort breathed. Harry could feel his pleasure, and had to keep his own revulsion down.

Bellatrix continued. Harry forced himself to watch all of it. This was what he was going to stop. The Death Eaters would remain without Voldemort, that was true, but they wouldn't have a strong leader to direct them in Voldemort's absence. And Harry's death would help him pay for the fact that he hadn't stopped this earlier.

There was shredding of limbs, and cracking of bones, and an intricate dance that Bellatrix performed, half like a snake herself, with the wizard's intestines, drawing length after length of them out of the wound in his belly. Voldemort's eyes were sharp enough that Harry didn't miss a detail. He left only when he was relatively sure the wizard was dead, and when his own reactions had grown too strong for him to control.

He made sure to seal the door into Voldemort's mind behind him, even before he raised his own Occlumency shields.

When he got his eyes open, he had to hurry to the loo. Most of what he'd eaten that day came up violently, and Harry was still dry-heaving five minutes later. Panting, he leaned his head on the cool wall and closed his eyes.

That's what I'm fighting. That's why I can't be allowed to falter, and I can't allow Snape to find out what I'm doing and stop me.

They're suffering. I hate it. But the suffering won't last much longer, I promise—either the pain I inflict or the pain I permit to happen. I promise. It'll be over very soon.

He had a date now, after all.

Harry sneaked quietly back into his room and listened to the other boys' breathing. Still as steady as ever. He drew the curtains tight around his bed, cast Lumos, and began reading the Potions book that would tell him how to make Medea's Draught once more. No use wasting the time, since he was awake anyway.

He would have to practice and become good at brewing the same way he'd become good at Occlumency. His own incompetence couldn't be allowed to stand in his way.

Nothing can, really.