|M-Day: On New Year's Eve, 2015, slightly over 90% of the world's mutant population were seized by some unknown force and died in a fiery explosion. The explosion had dire repercussions for the people around them, either killing them outright or causing area damage - some died driving or flying vehicles, were in areas that were delicate or flammable or explosive, etc, etc. One was an elected member of the House who took several other reps with her. It's estimated that of the close to 2 million mutants of varying power levels worldwide, less than 200,000 survived, most of those too low powered to be caught by the Cerebro scan conducted by Dark Phoenix. Their deaths caused an estimated 8 million dead worldwide and close to a trillion in property damage.
No one has any good explanation for what happened. Predictably, some said it was God judging mutants. Others think its a mass genocide at the hands of a mutant terrorist. Some posit its the natural end point of mutation. Surviving mutants have a conspiracy theory that it was done as an attack against mutants by humans. No one knows. With the lack of answers, fear and paranoia have driven most human/mutant integration processes and efforts into the background and the public demand is for protection against mutants. While not entirely outlawed, incidents of human on mutant violence have spiked and obvious mutants are subject to regular harassment and discrimination. Legislation calling for mutant registration, restriction of rights or outright segregation was fast-tracked in governments all over the world.
Characters who migrated from Phase 1 know the true cause of the calamity, but are unable to speak of it to anyone without risking the universe's destruction.