Remember that grey guy from the end of X-Men: Days Of Future Past? That was Apocalypse, the First Mutant who took over the world in an alternate dystopian reality where Charles Xavier was killed and Magneto ended up being the leader of mutantkind. The Age Of Apocalypse didn't pan out so well, with half the regular X-Men cast killed off or otherwise mutilated (Wolverine had one hand, for some reason). Thankfully some time travel shenanigans fixed everything, but this crossover remained a fan-favourite.