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[She looks a little amused.] This is strange to explain to you of all people.
[She leans back, trying to figure out where to start.] It's not all pretty, [Smirks] Professor X.
Magneto—Erik—formed a team of mutants, trained them, and named them the X-Men to fight for human/mutant peace and co-existence. Apocalypse had other plans. He is an immortal mutant. Very, very powerful. And very, very into Darwinism. I don't think he even cares about mutants. He just wants to see the strong destroy the weak and mutants are stronger, so... He started an army and a war.
What the X-Men do mostly is just try to keep the humans and ourselves alive and bring down Apocalypse any way we can.