In response to this prompt at the kinkmeme:

"Teamwork fics are win. Alone, Charles is a powerful telepath, and Erik is the master of metal and magnetism. But when Charles uses his powers to amplify Erik's, Erik is unstoppable, and vice-versa. Let's see an awesome boss battle where they come together on the same side and combine their powers, owning the shite out of the baddies.

Bonus if you use this scenario: Some really powerful mutant is raising hell in the modern age, and a reality-warping mutant brings both Charles and Erik forward into the present from the 60's, and to the shock of all the X-men and Brotherhood and stuff, they kick the crap out of the bad guy like it was nothing. Because I really like young!Charles/Erik in modern age fics."

Alas, I don't really like time travel, so this is set in First Class. However, as a bonus it is a nobeachdivorce!fic. It's nice to write a powerful!Charles fic that doesn't end in tears.

Onslaught is a shout-out to the comics where this prompt more or less happens only with more deaths and less cuteness.

Onslaught

He can't stop Erik, and part of Charles doesn't want to. The men on the ships tried to kill them. Not out of fear of these strange creatures, not out of ignorance, but because the men at the top, the men who knew what they were, who knew there were no enemies on that beach, but loyal Americans and a lot of frightened children. He can see Sean trying to hide behind Hank as the missiles fly towards them. Azazel tries to grab as many of them as possible, ready to teleport out. Mystique trying to get to him, to face this end together. He can feel Moira's mind frozen when she realises there's no one answering, that they are all being left to die.

The missiles stop, of course. They stop and start to turn to fire back at the ships, and Charles can't stop Erik, not with that helmet, but he will make sure Erik does not become the monster he has spent decades hunting. Charles reaches for the minds of the people on the ships. He can't stop Erik, but he can make sure no one gets hurt. "Wait, please," he gasps out, and snatches at Erik's hand to make him slow down.

It's like touching a live wire. Lightning bursts up from the contact and flies up Charles' arm like lightning. Erik cries out and the missiles drop into the water. He can vaguely hear Moira shouting something behind him and the children rushing forwards –

Stop.

And everything stops but them. Just him and Erik, who is still staring at Charles, eyes wide, breath coming in gasps. Lightning is crackling between them and when Charles raises his free hand to Erik's eyes, his fingers are webbed with light. It crackles and jumps when he pulls that absurd helmet off, trailing down to cup Erik's cheek.

It's all light, everything is light, as though the world around them has woken up and opened its eyes for the first time. Each breath tastes of fireworks. He can feel everyone, Moira panicking what is this what are they doing and the children's fear what's going on Magneto's gone mental what's the professor doing, and it's the work of a thought to soothe that fear, and show them that it's fine, that they're in control.

Erik is still blinking at him, and Charles realises he's feeling the same thing. That they've gone through serenity and out the other side; that Charles could make the whole world lie down and sleep, and that Erik could pull the Earth from its orbit if he wanted to, and part of him is laughing that it was all so easy and marvelling that he ever found raising a submarine difficult. He's smiling and Charles' heart breaks a little because this is the first time that Erik's not afraid anymore, because there's nothing to be afraid of. There's nothing anyone can do to them, and missiles are laughable when you can make the world dance around you.

Charles can see them through a thousand eyes, Moira and Sean and Angel and a thousand more from the ships. The two of them, standing so still on the beach, surrounded by a corona of light like a solar eclipse, lightning sparking off to ground in the sand around them, leaving shards of molten glass.

Erik turns back to the ships, and his eyes are glowing. He grins and sparks fly. He doesn't need the missiles. He could tear those ships in half himself. Or pick them up and drop them on the Pentagon. Or send them into space. He can feel the humming of every bolt and rivet, every metal filling in the sailors' teeth, even the pull of the moon on the tides, bringing the water to shore. Both their faces are wet with tears. This is so good. How could it ever have been so easy?

We need to do something. Erik's thoughts come in easily. The men are no threat to them, but they are not the only ones on the beach.

Charles smiles, touching the sailors' minds is nothing now, he just reaches out and they obediently leave their posts and climb into the lifeboats. You said it yourself, he whispers to Erik. We are the better men.

Not men. The answer comes, awed and delighted and still somehow smug. Gods. Gods among insects, Charles. He raises his hand and the lifeboats pull away. Brings it down and every boat sinks without a trace, breaking up the moment they hit the seabed. No one will ever find so much a wreck.

Charles wipes the minds of the men on the ships – he's doing them a favour, they don't need to remember how they almost slaughtered innocent children today – they will only remember going to bed last night in their bunks, and wake up on the lifeboats. Charles turns back to the beach, letting go of everyone's minds. Moira is holding a gun; she'd been planning to shoot Erik. The gun is gone the next instant, and Charles quietly wipes her mind as well.

She's an agent, she's seen too much. That's Erik's thought but Charles has to admit he's right. She knows who they are, what they can do and where they are based. After this the government of both great nations are going to be hunting them down with everything they have. It won't work of course, but it would cost the lives of everyone sent after them and put the children at risk. Much better for everyone if they can disappear.

Moira drops into the sand, unconscious. Charles lets go of Erik's hand, and the world shrinks down around them. It's like being doused with cold water and Erik shivers, shaking his head furiously as though to clear it. Charles blinks away tears. Becoming mortal again, becoming weak and small and helpless after touching godhood is impossible to describe. Like Lucifer banished from heaven. Their own private fall from Eden.

But it's still there, within their reach, only a touch away. They look at each other and Erik's still crying silently, staring at Charles with purest wonder in his eyes. His thoughts are bleeding out all over the place, so loudly Charles is amazed no one else can hear him, mourning and lost after the loss of such power, such joy. You're safe, he sends, smiling gently through his own tears. We're all safe. We never need be scared again.

Erik's head drops for a moment before looking up again, his eyes filled with the same wild happiness and wonder he had when he moved the satellite dish, only more, so much more now. You can't ever leave me, he's almost begging. Not now, not after this. This is meant; we are brothers, you and I.

I never will, Charles lifts his hand to touch Erik's face again, and sparks fly in the space between them, like lightning between two conductors, and we are more than brothers, I think. They don't quite touch but even the proximity is enough to make them feel light headed. It's exhilarating. We have to find out how we're doing this. Even in his own head the words sound giddy. Our powers must be creating some kind of feedback loop, playing off each other to amplify them this much –

Stop thinking Charles. Erik laughs in his mind. It's wonderful.

"Professor?" Alex is shaking, he's come two steps forwards but that's all he dares. "Professor X... Magneto -" His voice fails him, he stands there mouth open shaking his head in amazement.

The names sound wrong, mortal names don't suit what they've become now. He doesn't know which of them realises it, but it's clear as the seas around them, as the skies, as both their minds.

He smiles at Alex. "No." He can hear the word reverberating through all their minds as he speaks.

"We are Onslaught."