The Age of Apocalypse
In The Age of Apocalypse event that ran across all the X-titles in 1995 and 1996 (of which there were many, Astonishing X-Men, Amazing X-Men, Weapon X, X-Factor, Gambit and the X-ternals, X-Calibre, and Generation NeXt), a failed attempt to kill Magneto in the then present day by Professor X’s son, the mutant known as Legion, instead results in the untimely death of Professor X himself!
While this does lead to a long time villain Magneto renouncing his evil ways and eventually forming the X-Men in Charles stead, he does so too late to stop the early rise of Apocalypse, ten years earlier than in the main Marvel universe, and he takes control of all of North America.
Age of Apocalypse takes En Sabah Nur’s survival of the fittest maxim and takes it to its terrifying, mutant-flavoured conclusion. Apocalypse’s America is a land lorded over by the most “fit” alpha mutants, like Scott and Alex Summers, (Cyclops and Havok in the main timeline) who oversee vast populations of human slaves toiling all their lives for Apocalypse. It’s that or be sacrificed to the experiments of a dark, twisted version of Hank McCoy’s Beast, with those mutants who oppose Apocalypse suffering the same fate.
Many heroes of the mainline Marvel universe end up siding with Apocalypse in this dark reality. However thanks to visions by the mutant Bishop, himself a refugee from another, different, dark alternate future, disparate heroes from all of the comics, including Magneto and his wife Rogue, come together to attempt to defeat Apocalypse. All they needed to do was free the slaves, airlift refugees from America using Sentinel robots, and return reality to the original timeline in a massive crossover event.
