"Everyone has a weakness," Erik had once said to Charles, "And exploiting that weakness is fair game."

They had been wandering the grounds of the mansion on that hectic, emotional week before Cuba - this time was always meant to be a time for jogging and always turned into a time for discussion, for debate.

Charles had had issues with this statement of course. He had said, "Exploiting people is never fair game, Erik."

And Erik had laughed and said, "Charles, how many times have you exploited someone with your powers?" which, of course, Charles had had no answer to.

Charles remembers that conversation, once it is done. He thinks maybe, in time, Erik will understand. Charles was simply playing by his rules, that's all.

It starts seventeen years after Shaw's death, when Charles discovers a mutant who has the power to change reality. It is one of the most alarming powers Charles has ever heard of, so he is relieved to find that it is harboured within the body of a shy ten-year-old boy called Luka from Canada. Charles makes his own journey to meet the boy and his parents, which is rare these days. Usually he visits prospective students with at least one other person, Hank or Sean more often than not, but this child has the potential to be an specially delicate case.

The parents are terrified but desperate to do the best for their boy, which is about as hopeful a reaction as Charles could wish for. Luka displays his powers by making a hole appear in their living room floor, turning their dog into a surprised elephant and accidentally deleting his father from existence entirely. His mother screams, the boy panics and Charles tries his best to calm the situation, to persuade Luka that he can bring his father back again with Charles's mental help. After a few fumbled tries, in which Charles almost loses himself in the chaos flooding through the Luka's mind, the boy brings back the father, exhausted and pale, and the mother sobs and screams and shouts.

Luka is banished from the house along with Charles, and the poor boy cries all the way back to the mansion.

Charles tries to keep the potential of the boy's powers as under wraps as he can, until he knows just what Luka is capable of and - more importantly - what he wants to be capable of. But gossip flies easily around the school in normal circumstances, and a story about fathers vanishing from the world - in a school full of children who have more often than not been rejected by their parents because of their powers - is an irresistible tale. Soon everyone knows about it.

Someone must tell Erik, because Luka vanishes from the school mere weeks after his arrival.

Erik never resists a summons from Charles, even immediately after Cuba when he knew he would have to face the consequences of the bullet in Charles's spine. So when Charles calls, he goes, even though he knows it will be a meeting full of recriminations.

Charles does not disappoint. He sits at their usual chessboard in Central Park, upright and stiff in his wheelchair and ignoring the game entirely.

"I take it this is about the boy, then," says Erik as a greeting.

Charles's blue eyes are cold, but it has been years since he turned a warm expression on Erik, so Erik ignores it. "Luka was under my protection, Erik," he snaps. "You had no right to take him away. We promised each other, don't you remember?"

They had. Early on, when the Brotherhood and the X-Men were in their infancy, when Erik still had some mercy left inside him, they had met and agreed that they would never try to steal mutants from each other's groups. If a mutant was found by one group and decided to defect to another, that was their choice and neither Erik nor Charles would attempt to change their mind. They would never actively try to influence a decision by anyone. Neither of them want team members whose hearts aren't in what they are doing.

"The boy is different," Erik says smoothly. He taps a finger on one of the pawns in front of him, but doesn't move it. "The world is getting worse for mutants, Charles, you know that. The boy can change that. He could be the answer to all our problems. X-Men and Brotherhood alike."

Charles's jaw tightens. "He's ten years old, Erik!"

"Yes, and his parents just abandoned him," Erik argues. "You've seen how upset he is. If you ask me, he's halfway to the Brotherhood already."

"He's just a boy who has a power he doesn't know anything about," Charles snarls. "You're going to frighten him into doing something awful."

Erik half smiles, knowing it looks even more menacing under the shadow of the helmet on his head. "That's the idea," he says.

The X-Men call together an emergency meeting about Luka. They have spies in the Brotherhood just as the Brotherhood have spies in the X-Men, but these spies appear to know nothing. Whatever Erik is doing with the boy, he is doing it alone and telling no one.

Charles, tired and angry from his meeting with Erik, rubs his head and tries to marshal his thoughts together. "So the only way to Luka is through Erik," he finalises for them all. "And Erik has that blasted helmet which means neither I nor any other telepath can read him."

Storm runs a hand through her white hair wearily - she is young and new to the X-Men emergency meetings but has proven herself vital many times. "There must be another way to Magneto," she says. "He must have a weakness, something he's missed."

It is these words that hit Charles, it is these words that give him the idea.

He ponders the morality behind his idea for days, then spends another few days plotting the details, then spends a further few days worrying at the morality before he goes to the X-Men.

"I have an idea," he says. "But I can't tell you the details. I can only ask that you trust me."

They all do, unquestionably. Charles inspires trust. That's the problem, really.

They have shapeshifters in the X-Men, and though none of them are able to shift as effortlessly and quickly as Raven, they are good enough and happy enough to do the job. The next time there is an attack on an anti-mutant protestation by the Brotherhood, the X-Men fly in to defend them and Magneto finds himself face to face with what appears to be a tiny, terrified Jewish human girl.

"Help me," she sobs in Hebrew and Charles sees Erik freeze as if he has been shot. "Please don't hurt me, I'm sorry, please don't hurt me."

Erik pauses midway through an order, shocked to the core, and Charles curls his hands on the arms of his wheelchair, praying that this is going to work, that Erik is not too far gone.

The Brotherhood look to Magneto for orders. Magneto stands before the sobbing girl, irresolute, then raises his head. "We're leaving," he announces.

There are arguments which he cuts off with a mere look, and then the Brotherhood obey him as one, teleporting out or flying out while the humans grab at each other, sobbing in relief.

Erik shoots a look at Charles before he departs, and Charles manages to smile like he's proud of him and not disgusted at himself.

There is another summons from Charles, not long after the Incident, as Erik is now calling it in his head. He goes, suspiciously, to their chessboard in Central Park.

"I hope," he says, as he takes his place opposite a beaming Charles, "You aren't going to decide that one time means I've suddenly become a good person."

Charles smiles and says, "I wasn't thinking anything of the sort - I just wanted to play a game of chess with an old friend."

The word 'friend' makes Erik flinch inwardly. Charles hasn't called him that for years.

Charles's smile widens as if he can read Erik's mind, though Erik knows the helmet means he can't. "Black or white?" he asks.

Erik only just stops himself from rolling his eyes. "Black," he says.

They spend the afternoon playing chess. Charles says very little, but what he does say is lively and cheerful, two things he hasn't been in Erik's presence for years.

Erik is surprised by how much he enjoys himself.

The next summons is only a week later.

"For Christ's sake, Charles," Erik says, raging at himself for being unable to ignore a single request from the man, "You're only making this harder for yourself."

Charles smiles vaguely, setting up the board. "I don't know what you mean."

Erik blows the air out through his nose. Charles knows what he means. Charles isn't stupid. "This whole 'Erik is worth saving after all' thing," he says, "Is wrong. And you're going to realise that the next time the Brotherhood and X-Men fight and I do something terrible, and then you're going to be angry and betrayed and you'll push me away all over again, and we'll end up barely speaking just like before."

Charles cocks his head. "Is this your way of telling me you don't want to meet up anymore?"

Erik would give anything, anything at all, to be a mind reader at this moment. Charles isn't this stupid, so what is he doing?

He could walk away now. Just stand up and walk away.

He sighs. "Set up the damn board."

The third time, Erik looks up from the chessboard to find Charles's eyes on his, level and bright. It feels like Charles is peeling away layers of Erik, piece by piece and with ruthless determination.

For a moment, Erik can't breathe.

Charles smiles, a slow, stretch of a smile. "Willing to concede defeat?" he asks.

Erik opens his mouth but nothing comes out.

"The game," Charles says patiently. "Have I flummoxed you?"

Yes, Erik thinks. He pulls himself together and looks back down at the board. "Of course not," he says, and promptly captures Charles's knight.

Erik decides, after that, that he won't answer any more summons. He'll be busy instead. Charles is, all at once, too close to and too far from what he is trying to achieve in the world. They will never agree on what they should, and they will never be friends, not really, so why do they keep trying? Erik has other things to do with his time.

The next summons mentions dinner. It changes things. Erik decides he won't go.

And hour before the meeting time, Erik stares at the letter, grabs his helmet, puts it on, and leaves.

They eat dinner in a restaurant and no one stares at the man wearing the helmet, so Erik can only assume that Charles is deflecting people's minds away from noticing it. He glares at Charles.

"You don't need to do that, you know. I don't care if those stupid humans stare. They'll all be gone soon anyway."

It's more than he meant to say, and Charles blinks for a moment. Erik blames it on the food, the wine and Charles's eyes, which only seem to have got bluer over time.

"We haven't got much time," Charles tells the X-Men the next day. "I think Erik's getting close to cracking Luka."

Hank rubs his eyes. "Luka won't want to work for the Brotherhood after this, at least," he says.

"There won't be a Brotherhood after this," says Charles, dread pumping through him. "If I'm right, Erik's going to use Luka to end the existence of humans everywhere."

Storm stares in horror. "Can he do that?"

"It depends on Luka's power," Charles says. "If Erik uses something like Cerebro, he could further Luka's reach to encompass the whole world."

"We need to do something!" Sean splutters.

"I am doing something," Charles insists tiredly. "I thinkā€¦I think I just need to do it a bit faster."

And hope it works, he thinks. God, I hope this works.

"Seventeen years," Charles says on their next chess date - and Erik is not thinking of them as dates, he really isn't.

Charles doesn't need to say what he means by that number, because Erik has been counting too. He moves his rook in place of an answer.

Charles huffs out a laugh, eyes distant in memory. "Do you remember," he said, "When we pushed Sean out of that first storey window?"

It feels like centuries and centuries ago, and yet Erik can remember every aspect of that time he had with Charles before Cuba split them apart. "Yes," he says.

"It was fun, that week," Charles says with a crooked smile. "I know it wasn't meant to be, that we were worrying about Shaw and everything and we didn't even know if we'd survive, but it was fun sometimes, I thought."

Erik can't speak. There is something in his throat.

Charles turns level eyes on him, as if he is about to impart something of great importance. "I do miss you, Erik," he says.

The next summons is to Charles's bedroom. Erik stares at the letter for hours and hours, but he's never turned down a summons from Charles. Not ever.

He gets in through the open window, flying softly into the school like some kind of mutant burglar. Charles is sitting in a chair near the window, looking about as terrified as Erik feels.

He drops to his knees in front of the man. "Charles," he says desperately. "Seventeen years Charles, why now?"

Charles reaches down with both hands and cradles Erik's helmeted face gently. "Because you let her live, Erik," he says.

There is no point explaining that it was merely once, that Charles cannot delete a lifetime of being a monster and start Erik all over again. Charles will only ever hear what he wants to hear. And there is no point saying this will all end in disaster, because they've always known it will and it hasn't stopped either of them yet.

Erik is not strong enough or good enough to push Charles away because he doesn't deserve him. He never has been. He leans up and kisses Charles instead.

Seventeen years and it's like their lips haven't been parted one day. Erik's mouth knows just how to slant itself across Charles's, his tongue knows just how to curl in the way that makes Charles moan. Charles clings to Erik's jacket, pulls him up so that Erik is in a kind of crouch, and its uncomfortable and unfamiliar because Charles can't move his legs, but nothing right now would tear Erik away from Charles's mouth, so he merely plasters himself against Charles and lets comfort be damned.

Charles breaks away from the kiss with a quiet sort of whine. "The helmet," he mumbles. "It's like kissing a black hole, Erik."

Erik feels himself pause. The helmet feels like part of him now - he hasn't taken it off once in seventeen years. He touches the edges hesitantly. "You won't - " he starts, then looks at Charles's face, into those blue eyes that were once so cold and now are full of such warmth, and the words die on his mouth.

There is no way he could not trust this man.

He pushes the helmet off his head, letting it fall to the floor carelessly. Charles smiles, places his hands on either side of Erik's face, drawing him back in.

"Lovely," Charles murmurs distantly before he kisses him. "Yes. Perfect."

When Erik makes his way to the abandoned research station at Alkali Lake, where he's been keeping Luka, and finds it empty, he doesn't understand at first. That is how much being with Charles has scrambled his brains - he doesn't even comprehend what could have happened. Then he understands all too well.

For the first time in their history, it is he who summons Charles.

Charles comes to the meeting point - not Central Park, not dinner, not Erik's room but a simple street in a simple lane that could be anywhere at all. There is no one around, which Charles has obviously arranged. He is pale and his eyes are wet. Erik has his helmet on, though he knows the damage is done.

"You were the one person I trusted," he says. "Out of everyone I know, I only ever trusted you. And you are a telepath. Do you know how much faith I had to have in you to place such trust in you?"

Charles winches, though his voice is perfectly steady. "I know," he says. "But you were going to kill them all, Erik."

Erik sets his jaw, lets the ugliness back in. "And I still will," he decides.

The tears that Charles clearly hasn't shed until now glimmer at the edges of his eyes, some sliding down his face. "I know you still have good in you," he says, his voice cracking. "I didn't make that bit up. Please don't forget that, Erik."

But it is already gone, that little shred of hope that Charles somehow, after all that time, still managed to drag out of the depths of Erik's psyche. Erik shakes his head gently. "Why did you do this to me?" he asks.

The tears are still on Charles's face but he is calm when he says, "Because everyone has a weakness. And yours was me."

Erik pulls off his helmet one last time, and then faces Charles and lets his thoughts wash through him - everything he had felt in their last few weeks together, all the tentative happiness, desire, confusion, hope, trust. And then he shows Charles the pain he feels now, the tightness in his chest, the old murky darkness flooding his mind again. He shows Charles his heart breaking.

Charles's face crumples. He buries his face in his hands, shoulders heaving.

Erik puts the helmet back on and walks away. His work is done.