Storyline
Main Article: Legion Quest
Legion (David Haller), a psionic mutant on Earth and son of Professor Charles Xavier, travels back in time with the intention of killing Magneto. However, Legion traveled to a time when Magneto and Xavier were still friends. As Xavier dies trying to protect Magneto, Legion vanishes, and a new time-line is created. The only person aware of how history has changed is Bishop, a time traveling mutant who followed Legion.
Because of Xavier's sacrifice, Magneto comes to believe in his late friend's dream of a peaceful coexistence between humans and mutants. Apocalypse, an immortal mutant villain, was monitoring the fight. He chooses this moment as the perfect time to begin his world conquest, which did not happen in the regular Marvel Universe for another ten years.
Magneto assembles the X-Men just as Apocalypse begins his war. Despite the X-Men's resistance, Apocalypse conquers all of North America and eventually mutants are considered the ruling class. Apocalypse also initiated a genocidal campaign called "cullings" killing millions of humans. To further ensure that no one was left to challenge him or undo the circumstances that led to his reign, he had everyone with telepathic or chronal abilities hunted down. Meanwhile, the changes in the time-line result in a destructive crystallization wave created by the M'Kraan Crystal.
Read more about this topic: Age Of Apocalypse