The starting point for House Of M saw Scarlet Witch suffering a mental breakdown and using her reality-warping powers to try and resurrect her dead children. In doing so she created a horrifying alternate world presided over by Magneto, where mutant life is sacrosanct above all others, eventually leading to most of humanity dying out. That was less fun. Then it all got undone by Scarlet Witch undoing her spell which aimed to give the Avengers and X-Men their heart's desire, creating that world, instead bringing back the regular reality and significantly reducing the mutant population. Woops.