That’s Legion and, in an effort to please his Dad, he went back in time and tried to kill Magneto before he became evil; but ended up killing Charles Xavier instead.
Before he’d ever had Legion.
The resultant anomaly created a Marvel universe where Magneto, instead of Xavier, formed the X-Men and Apocalypse won, enslaving the world and remaking it to his liking (which somehow involved lots of body modification and big shoulder pads). It was brutal and more than a little bleak. It’s the seeming hopelessness of the situation the AoA heroes constantly find themselves in that makes this a lasting favorites among Marvel-philes. Not only was it a shake up of decades-old team dynamics, not only was it a reshuffling of everyone’s values and personalities, it was a nigh-near unwinnable situation where anyone could be snuffed out at a moment’s notice. That was what kept me collecting nearly this entire event when it came out, the sense that the safeties had all been turned off and my favorite characters were all in the crosshairs.
As an added bonus, the new universe meant that the artists could pull out all the stops and redesign to their hearts’ content. Some of my favorite characters (Blink, Holocaust) and re-designs (Iceman, Sunfire) came out of this event and are still used (or at least referenced) today.