Professor Xavier's son Legion (David Haller) went back in time planning on killing the mutant known as Magneto to prove to his father that he really loved him and to protect everyone from the madness that would become of Magneto. X-men had atepmted to stop him but did not. Legion had confronted both his father and Magneto. Readily Legion prepared a psychic knife that would kill Magneto, he lunged towards him but to his surprise Xavier was not willing to let Magneto die. Xavier lunged between his son legion and Magneto, dying in the process. At that moment it changed the course of time and space from what Altering Reality.
In the new reality, new world, Apocolypse is the ruler of America. New York City Is now Apocolypse Island, and all humans are sentenced to slavery! Only the most powerful mutants being Holocaust, Abyss, Sinister, Cylops and the Dark Beast survive to reign alongside the High Lord En Sabah Nur also known as Apocolypse!
In this different nand new world the X-men were founded by Erik Magnus Lehnsherr, better known as Magneto with Rogue by his side rather than the original but dead founder Professor Xavier. The X-men roster has changed slightly and their quest for peace remains unchanged. due to the drive instilled in Magneto by a man he once called a dear friend-the late Charles Xavier.
FULL STORY IN THE AoA BIO