The man known as En Sabah Nur was one of the earliest mutants on the planet. Twenty years ago, Apocalypse, along with his trusted ally, Sinister, witnessed the first televised proof of the existence of other super-beings when Legion accidentally murdered his father, Charles Xavier. Apocalypse, a strict believer in the survival of the fittest, launched his offensive against humankind. With Xavier dead, and Apocalypse beginning his offensive ten years earlier than he did in the Marvel Universe, there was no one to stand in his way.
Eventually, Magneto created a team of mutants called the X-Men. By this point Apocalypse had already gathered his Horsemen. Originally, the Horsemen were his lieutenants, and he had many of them. After Apocalypse took over all of North America he announced the War of Secession, a battle between all of his Horsemen; he let it rage until only four Horsemen remained.
As time moved on, Apocalypse defeated most of the world's governments. The only remaining threat to his rule was the X-Men and the Human High Council in Europe. It was only a matter of time before the entire world would be his.
Apocalypse planned a surprise attack while he negotiated a peace treaty between mutants and humans. Named the Kelly Pact, it called for peace and the end of genetic testing, something Apocalypse never planned to do.
However, Apocalypse did not realize his kingdom was crumbling around him. His greatest and oldest ally, Sinister, had deserted him. His most powerful Horseman, Mikhail, had gone rogue. And Abyss and Holocaust were engaged in personal battles that distracted them from the big picture. Apocalypse had surrounded himself with powerful men who were proving to be less than reliable.
Apocalypse finally learned the whereabouts of Magneto and captured the X-Men's leader. A lesser player, the X-Ternal traitor Guido Carosella actually captured the most important piece of the puzzle, the M'Kraan Crystal, and delivered it to Apocalypse. Magneto planned to use it to change the past and ensure the Age of Apocalypse never came to pass.
The X-Men teleported into Apocalypse's New York City base and attacked his forces. Magneto was freed by X-Man, Sinister's genetic creation. Magneto fought Apocalypse and tore him apart by using all the magnetic powers at his disposal.