The Great Reality Migration

This is basically to explain to anyone what exactly The Great Reality Migration was, what happened during it, and what impact it has.

The Background (AKA The Y-Men Vs. The X-Men)
Alpha and Omega are two reality warpers who have chosen to amuse themselves with using the x men in a manner that a child might use toys or in Omega's case more like how a child might interact with a colony of ants using a magnifying glass.

Omega gathered her own team of X-Men who she called the 'Y-Men', which consisted of: Omega sent her 'Y-Men' to fight the X-Men in a battle royale governed by a set of rules which Omega took great joy in exploiting the hell out of to her own benefit, while Alpha actually tried to follow them. At stake was the entirety of Earth 1235. The key role was that Omega and Alpha could not directly affect the events of the battle and more consequentiaslly what happened in the battle could not be done after the fight was over.
 * Earth 1235's version of Rift - who was a coward and functionally a living bag of many things namely guns.
 * A male version of Abigail from an unspecified reality who could in addition to separate his astral self from his body also have his body for the duration carry out a fairly simple course of action such as haphazardly try to repeatedly hit someone
 * An alternate version of Dr. Pym who could change his shape as well as that of others
 * An alternate version of Nightshade (Rose) and an alternative version of Pedro (Thorn) who had gotten together after Thorn's version of Sargotha had died giving birth to their version of Lance (who had also died) and then had proceeded to basically kill those they considered to be a threat to the environment namely everyone else, by releasing toxic spores into the atmosphere.
 * An alternate version of Fire-Cat who rather than be reverted to normal by the x men had instead elected to upon being corrupted by the Dark Child persona of Magik, kill Magik and then Belasco and make herself the queen of Limbo and not the party game type of Limbo either.
 * An alternate version of Harold named Henry

Due to a series of events set in motion by the institute's pool water being irradiated and then used to douse Hellraiser those giving him the world's worst case of full body cancer, Alpha called off the fight and conceded the fight to Omega and her Y-Men. Though he proved to be just as much of a dick to Omega as he had been to the x men, by pointing out that the stakes had been control of Earth 1235, which he happily handed over full dominion of to Omega, before proceeding to leave with the actual prize of the fight: the x men.

Fitting Square X-Men into a Round Reality
The condensed version is that Alpha put the x-men into a sort of stasis, while he looked for a suitable alternative reality. Upon finding a new reality (Earth 12357) Alpha proceeded to make room for the x men and all their associates by baically killing Earth 12357's x-men and all their associates, a fact that was decidedly noticed by the entire population of Earth 12357 as it resulted in the simultaneous death of thousands of people for no explainable reason.

There is a two week gap in time between the X men disappearing from Earth 1235 and then basically waking up with in most cases their psyches inside what were to one extent or another a body that was a composite between their bodies in Earth 1235 and their bodies in Earth 12357. The most profoundly affected individual being Richard who found that his new body could not process cooked food and he had an intense drive to eat living animals which he has since tampered down and substituted with a diet of mostly frozen rat corpses. His face also can split open to facilitate the swallowing whole of large 'food'.

The act of relocating the x men proved to be even too much for Alpha who found that though the reality was the right fit for the x men it presented a distinct problem for him namely that Omega was no longer around to balance out all his 'good' and so he was worst to balance himself out by being destroyer as well as a creator, but with only his ability to create. So far this has manifested mostly in the form of creating black holes in distant and remote corners of the universe to obliterate entire galaxies.

Once he had revigorated his ability, he finished the job and made a vast majority of the population forget about the whole global simultaneous death event with only the x men retaining all their memories and thus who he knew would one day becomes x men retaining vague memories of the event.