Severus waited for the children to leave the room before he confronted the headmaster himself. Much as he loathed to admit it, the little brats did have a point. Time-travel was a dangerous, restricted form of magic and the headmaster seemed hell-bent on letting a group of wizards a millennium out of time wander freely around his school. He waited a minute before they had all trudged from the office, muttering suitably mutinous complaints against the 'greasy bat of the dungeons', before stalking over to the door and swinging it open. Potter, to his complete lack of surprise, was crouched next to the keyhole making a masterful attempt at pretending to be tying the laces of his bedraggled trainers. Severus narrowed his eyes at the boy and pointed him down the staircase. He ignored Potter's permanent petulant glare and watched as the boy stomped down the staircase. Throwing up a quick muffliato in case he decided to try his luck once again, Severus stepped briskly back into the headmaster's office, pulling the door shut behind him until it closed with a heavy thud. This was not a conversation that he particularly wished to have with Dumbledore.

"You think that I am making a mistake," the headmaster said simply, as soon as Severus had placed himself in the empty chair by the desk. Of course he knew what he was going to say, Snape reminded himself snidely – the man is, after all, omniscient.

"I do," he replied calmly. Although 'mistake' was a large understatement, 'cataclysmically ridiculous decision that may lead to the eradication of the wizarding world as we know it' was far more appropriate. "Headmaster," he began, reaching for whatever patience he had left at the end of a long and tortuous day, "Surely we cannot allow these time-travellers free access to our school."

"I think you'll find, my boy," the headmaster replied, with an odd smile on his face, "that it is in fact, their school."

Severus was in no mood for word games with the headmaster. There was far too much at stake for them to distract themselves with the curious improbabilities of temporal displacement. Sod who owned what when Godric Gryffindor had appeared in the entrance hall with his malicious prick of a father, dragged through time by the mad magical experiments of Lady Ravenclaw and her companion Helga Hufflepuff. They clearly hadn't known where they were and had no recollections of being in, let alone founding, a school, or of knowing a Salazar Slytherin for that fact. Hogwarts wasn't theirs yet and that was the end of the matter, as far as Snape was concerned.

"Headmaster, this is serious!" Severus found himself hissing. He took a deep breath and attempted to control himself. It was most unlike him to let his frustration and stress get away from him so easily. If such a thing were to happen before the Dark Lord, the consequences would be distinctly unpleasant.

Dumbledore shifted forwards in his seat, stapled his fingers and stared down at one of the spinning trinkets on his desk. Severus noticed that he seemed to have far fewer of them this year.

"Do not think, my boy," the headmaster began, "that I am taking today's developments lightly." His words were light, spoken with a careful delicacy that raised the hairs on the back of Severus's neck. He was entering into dangerous ground, but he had to make the headmaster see sense. The older man had been making increasingly disturbing decisions of late and disappearing from the school at odd hours. He wasn't sure if it was the effects of the curse at play, but Severus was becoming concerned that the headmaster was losing focus on anything that wasn't related to the war. He quietly worried that the only thing that would now motivate the headmaster was the destruction of the Dark Lord. That boded well for the war, but ill for the school.

"Then why allow any of this?" Severus bit out "Surely someone of your talents must know something of the theory of time travel? You must do something to get them back to their own time!" Even as he said it, he knew that he had pushed too far. The headmaster's head snapped up, colour rising in his cheeks. He pinned Severus with a fierce stare.

"What would you have me do?" Dumbledore thundered. It was very rare that the headmaster raised his voice at all and Severus froze at the unexpected change of tone. Perhaps the headmaster was more disturbed by this than he had believed. "We are at war, Severus! You do not need me to remind you of that fact." Severus frowned in irritation; no he most certainly did not. Dumbledore eyes shone brightly enough to scald, as he continued. "I have a responsibility to the students of this school and to the rest of our society. Voldemort must be stopped."

Severus sat back as the headmaster stood up from his chair and beginning to pace the span of his office, continuing his impassioned speech. Severus did not think that he had ever seen the headmaster this affected.

"I am an old man, my boy," Dumbledore stated, lowering his voice with an attempt at restraint that was clearly causing him great difficulty, "and I am dying. I am on borrowed time and I must make the most of all that I have left to impart to young Harry all that he needs to know to win this war." The headmaster had calmed as he spoke, his voice dropping with a weighty sorrow as he spoke about Potter's inevitable conflict with the Dark Lord. Severus felt the familiar sneer rise up at the thought of James Potter's son. As always, the Chosen One would be at the top of the headmaster's priorities. The rest of them all be damned as long as Dumbledore's sacrificial lamb is properly prepared to die as demanded.

He frowned to himself, unsure as to where Dumbledore was going with his lecture. It was all well and good bringing up the war, but it had precious little to do with the unexpected guests in the castle and why the headmaster had not made any attempts to send them back where they belonged.

As if reading his thoughts, which Severus knew was impossible (he had exemplary Occlumency shields) the headmaster smiled at him. "Nor am I omnipotent, Severus, despite what the Daily Prophet is currently peddling," Dumbledore half chuckled. Severus met the headmaster wry smile with an even expression. "Even I must be allowed to have difficulty understanding magic that perplexes even the great Rowena Ravenclaw."

"So we are to do nothing, then?" he found himself saying, voice drier than the bones he used in his potions. "We are to allow these time-travellers free reign in a school. I apologise headmaster, but I do not share your confidence that children will be capable of understanding the intricacies of causality." He winced as if he had just eaten something very sour. "Miss Granger has a point" and that had hurt him to say. "'Terrible things happen to wizards who meddle with time' and these are the founders of this school, one of the premier institutes of magical education in the world – they have influenced society more than Merlin himself! One false step and we could destroy the wizarding world as we know it. If they so choose, Hogwarts herself may cease to exist." Severus felt himself paling at the sheer enormity of the danger they were in. "There is precious little purpose using all of your time educating the Chosen One if we unintentionally rewrite the whole of known history through sheer negligence!" Severus had not realised that he had started shouting, or that he had stalked the length of the office, until he was a foot away from the headmaster and staring directly into his damned twinkling eyes.

"I agree, Severus," Dumbledore replied, "but I do not believe that such a catastrophe will occur. I do not profess to understand the intricacies of time travel. Some mysteries, I believe, should remain unsolved. But I do believe that we are bound within certain parameters and that that which we seek to change is often brought to pass despite our will, or even our better judgement. In short, my boy, I believe that our visitors were always meant to find us and that any consequences of their presence here have already been played out through time." His tone was much too reasonable and expression far too understanding for Snape to be able to hold onto his anger, or his panic. He felt it washing away from him, to be replaced by sheer resignation. He watched as the headmaster returned to the chair behind his desk. Fawkes trilled compassionately from his perch, before flying over to rest on the headmaster's shoulder. Severus couldn't help but feel the calming wash of Phoenix song settle in his bones, and began to think that he had, perhaps, been overreacting; he lowered himself into the chair opposite the headmaster and took a deep, calming breath.

"Headmaster," he began, unable to take his eyes from the gentle, methodical way that Dumbledore was stroking the bird's feathers.

Dumbledore looked up at him, piercing him with those startling blue eyes. Suddenly, Severus couldn't think of a single thing to say. The man in front of him looked old, far older than Severus had ever thought possible. His skin was sallow and paper thin from the curse that wracked his body, but it was his eyes that made him ancient. There was a terrible weight that rested behind that perpetual, irritating twinkle. Even though Severus had often wished for a greater level of sincerity from the headmaster, he found the absence of his typical joviality incredibly jarring. He felt his throat tighten in dread.

"I understand, my boy," the headmaster sighed. He shuffled some paperwork on his desk and cast Severus a look from the corner of his eye. "It is human nature to react protectively when one's home is under threat. Hogwarts was the first place that you truly considered home, was it not?"

Severus did not miss a beat.

"You misunderstand my motivations," the lack of an honorific was the only indication Severus allowed, to convey how irritated he was at the headmaster's attempts to empathise. Dumbledore hummed in curiosity and he continued. "The school itself is obviously of great concern." He chose to ignore the headmaster's small smile at his carefully constructed understatement. "But there are a great many other consequences to consider if you are wrong. Not the least of which, as I have already noted, is the eradication of wizarding society itself."

"Not the least of which?" Dumbledore replied with amusement, "Next to such a calamity, my boy, what further catastrophes, as a result of my inaction, do you possibly perceive as a worthy source of your distress?"

Severus took a sharp breath. As always, this was the difficulty when speaking with the headmaster –the greater good must always come first.

"I thought that enabling the continued enslavement and abuse of a child might be a rather worthy reason, headmaster," Severus hissed, shining a lumos on the elephant in the room that the headmaster had allowed to be beaten about in his office, and sent to the gentle hands of a sadistic squib. A dark delight purred in his chest as Dumbledore physically flinched, all amusement dropping from his face as he fell into an expression of intense pain.

"Ah," Dumbledore began, "As always, Severus, you play the better man and force me to see that which I have overlooked. You and young Mr Potter both." Severus frowned, he was most definitely not the better man; he was not even a good man. He was bitter, cruel, and uncompromising, yes, but he was also, thankfully, nothing like James Potter's arrogant spawn. Dumbledore could at least refrain from insulting him. "I am afraid that there is little that we can do for our young friend," the headmaster continued, "their society is not our own. We cannot arrogantly claim moral superiority and restructure their lives on a whim."

"I do not think that rescuing a child from slavery is a mere whim, headmaster," Severus answered tightly. "If we do no nothing, then we are permitting abuse to continue." He refused to meet the headmaster's eye, knowing that he could not continue this discussion if he had to meet that knowing look. "It was odious enough that we allowed a child to cower on his knees in front of us."

"The child is held under a life debt, my boy," Dumbledore stated seriously. "I trust you now understand why I once went to such great lengths to impart the significance of such a bond to James Potter. To save the life of a friend requires no payment, it is an act of love, which is a magic far too often overlooked. To save the life of an enemy, however, is an act of sacrifice, and so requires payment. It may be morally repugnant to us to force another into servitude to appease such a debt, but the magic itself is not so discerning."

Severus looked up in shock. His throat tightened and his ears drummed with pounding blood. He looked down to see that his knuckles we clasped pure white around the arms of the chair. Forcing himself to take deep breaths, he willed his fists to unclench. He had been so close, then? So close to- to that? Forced servitude at the hands of Potter? All because he'd stopped him from seeking out a werewolf on a full moon? He had never felt so sick in his life. He would have suffered more under Potter, with Black eagerly watching on, than he ever had serving the Dark Lord. There were few things in life that Severus was certain of, but this was one of them.

"I believe that you understand," Dumbledore said gravely. "I do not mean to cause you distress, my boy, but you must trust me when I tell you that I cannot free that boy from his bond, nor can I control how Lord Gryffindor commands his service, even as headmaster of this school. You saw how he was treated, to interfere much more would only make things worse for the boy." Dumbledore paused and sighed deeply. "Magic allows us the freedom to do many marvellous things," he mused wearily, "but we are still subject to its laws and absolutes."

Severus sighed and let his head fall into his hands. He remembered, suddenly, walking home from the pictures with Lily one late summer evening, back when they were both young enough to pretend that things like blood status and social position didn't matter. There was a dead pigeon lying in the street, in the gutter by the hairdressers. Just a dead pigeon, vermin even. It was something they'd seen a hundred times before and by far the least disturbing thing that either of them would end up seeing, before Lily Evans was ripped prematurely from his world; but Lily couldn't get it out of her head. They were halfway down the street before she burst into tears and ran back, feet pounding on cobbles that threatened to turn her ankle at any moment. Severus had followed in alarm, bent double and gasping for breath when he caught up with her.

When he realised why she was so upset, he'd nearly burst out laughing. He had always been far crueller than she was. They stood there in silence, as the postman cycled past for the last collection of the day and the streetlights began to flicker on. They had been staring at the corpse for what was definitely an inappropriate length of time before Lily finally sighed, nodded, and walked away. As they left it behind and ambled home in the burnt orange glow of the setting sun, she had asked him how come, if magic was so incredible, it couldn't stop horrible things from happening. She looked so serious, tear-filled eyes looking so startlingly, beautifully green, that he almost lost his breath. "Some things," he had replied shakily, as he tried to remember how to speak, "are beyond even the control of magic." She had shaken her head, stating that nothing could be beyond magic if you cared enough to fight for it. She refused to accept his explanation as they trudged home, wrinkling her nose and setting her face into that stubborn expression she had that could bring a priest to swear. She'd denied any attempt to convince her otherwise, even as he hugged her goodnight on her doorstep, illuminated by porchlight, with her harpy of a sister glaring at them from the upstairs bedroom window. When he finally got home, his father had beaten him for his lateness. He hadn't even cared.

"No," he said suddenly, forcing himself back to the present. "I refuse to believe that some things are so far beyond our control. There must be a way out of such debts, surely the magic cannot allow them to continue indefinitely? The payment must somehow be judged complete" He met Dumbledore's eyes coldly, daring the older man to challenge him. He never felt more adamant than with the fire of Lily's conviction raging through his veins.

The headmaster sighed and nodded his understanding to Severus.

"I wonder," Dumbledore murmured quietly, as he stared down at the ruins of his arm, "what is the price of a life?" They sat in silence as Fawkes began to sing gently. The ticking of the clock on the cluttered desk sounded a hypnotic beat to the bird's melody. Finally, the headmaster shook himself and smiled in gratitude to Fawkes, before turning his attention back to Severus.

"Indeed, my boy, you are right," he smiled ruefully. "You must permit an old man his mistakes; I find myself making more of them recently than I ever did in my youth. Or perhaps I am just better able to recognise them for what they are." He looked Severus dead in the eye, as he continued. "There was once a boy, an orphan, brought up in a muggle orphanage. I gave him his Hogwarts letter, introduced him to the magical world. I sensed something in him, a darkness, a cruelty even, from the very first moment that I met him." Severus felt a shiver ripple through him, as the story continued. "He passed through these halls like a prince, bringing all those around him under his thrall, and yet I did nothing. He committed atrocities within this school that lay hidden for decades, and yet I never noticed. I do not pretend to be any more than a man, Severus, even if I am, if I say so myself, a lot cleverer than most. But, as I look back, I believe that I should have done more to stop him. Perhaps it is sheer hubris to suggest that any one man could have prevented the rise of Lord Voldemort, but I cannot help but hold myself responsible. Had I known what he would become, I would have done everything within my power to stop him." Dumbledore paused then and took a deep breath. "You know who this boy will become, Severus."

Snape nodded tightly, how could he not? It was a startling revelation, but it had hardly been a strenuous leap of deduction from the pitiful slave Sal, to the absent Salazar Slytherin.

"You are not the only one, I am sure. If young Mr Potter and his friends have not solved the mystery yet, they will soon. As, I'm sure, will a silent majority of our students. It is exceedingly difficult to keep a secret at Hogwarts, despite one's best attempts," Dumbledore grimaced and Severus recalled how certain 'secrets' had, over time, become common knowledge around the castle; he was fully convinced that Dumbledore's 'best attemps' varied much upon both his whim and his desire to fuel Potter's suicidal thirst for reckless self-endangerment.

He waited patiently for the headmaster to continue.

"Perhaps you may help him, or perhaps this will be one task too great for even a great many men to accomplish. We are working against Fate, after all, and she is known to be a fickle mistress," Dumbledore smiled, but there was a great deal of pain behind his eyes. "If I am right with my theory that the presence of our guests will not disrupt the timeline, some things will already be set in stone. The child you seek to help now will still become the Salazar Slytherin whose teachings have poisoned our society. If I am right, which I believe I am, then whatever you do, for good or ill, will not change anything for us in our present- or…" he paused and looked at Severus with more compassion than he had thought it possible for one man to possess, "or, indeed, our past, my boy."

Severus nodded, simply.

"I understand," he said quietly, standing and pushing his chair neatly into the desk. He nodded politely at Dumbledore. "Goodnight, Headmaster."

He walked from the room, followed by the murmured response. He was confident, after thinking it through, that the headmaster was right and that none of their actions would have any detrimental effect on the timeline. This appeased his immediate concerns about letting the founders of Hogwarts loose in a castle of ignorant brats, but did little to appease his conscience. He might not be able to change the past and stop the rise of blood supremacy that led to lifeless green eyes and heart wrenching sorrow. He might not even be able to do anything about the life debt, but he was certainly going to look into it; he was damn well going to try. He very much refused to allow a child to walk around Hogwarts as a slave and not a student. Until he found a way to break that boy out his bond, he was going to do his best to keep him safe from the worst of his so-called master, and try to find a way to impart some education on him before he left the school. He'd been doing nearly the same thing for Potter for years; it would hardly be anything new or strenuous. He smiled bitterly to himself and began the long walk to his chambers. Severus cared enough to fight for this child; he'd find a way to make the magic work. With that thought, he smiled genuinely for the first time in a very long time. He walked back through the darkened corridors, basking in the burning power of Lily Evans's conviction.