"Age of Apocalypse" is a 1995 crossover from Marvel Comics crafted by a massive creative team including Scott Lobdell, Mark Waid, Chris Bachalo, Adam Kubert, Roger Cruz, Jeph Loeb, Fabian Nicieza, Ron Garney, and more. The storyline starred Legion, the Omega Level mutant son of Charles Xavier, who previously traveled back in time in the "Legion Quest" storyline in order to kill Magneto and repair his fractured relationship with his father. In the story, Legion ends up 20 years in the past, meeting with Magneto before his villainous turn. As he's about to murder him for spoiling his father's dream, Professor X jumps in the way of his son's killing blow. As a result, Charles saves his longtime friend and sacrifices his own life to protect his future rival.
The death of Professor X had severe ramifications for the X-Men's place in the Marvel multiverse, creating an all-new timeline where the villain Apocalypse was able to take control of the world. The X-Men undergo drastic changes in a world ravaged by Apocalypse's rule, with the mutant Bishop being the only person in the entire reality who is aware that the world isn't as it should be. As a result, Apocalypse ended up killing millions of humans while recognizable mutant heroes and villains received significant changes to their appearances and personalities in a timeline filled with death and destruction.
The dark timeline of "Age of Apocalypse" makes a lot of sense as a potential storyline to reboot the X-Men for the MCU. Instead of killing the Marvel Universe as he did in "Deadpool Kills the Marvel Universe," killing Professor X alone could potentially accomplish the same goal of creating a new timeline, one designed to introduce the X-Men to the MCU.
