The post-credits tag for X-Men: Days of Future Past barely qualifies as a spoiler—it's more just a tasty morsel for comic book fans, setting up the next movie's villain and probably leaving more casual viewers baffled. Our glimpse of Apocalypse, who had been announced as the centerpiece of the next film by director Bryan Singer before Days of Future Past was even released, doesn't make him look too scary. He's a pale kid, building the Egyptian pyramids with his brain. What's so terrible about that? We love the pyramids!
But if Apocalypse is anything like his comic book iterations, X-Men: Apocalypse is going to be the first X-Men film to pitch its heroes against a classic comic-book villain. The kind who's eeeeevil and indestructible and can only be taken down in an epic slugfest that will eat up the last act of your movie. While the seven X-Men films released so far have featured some very bad men, there's never quite been the kind of super-powered "bad guy" that's more commonplace in, say, the world of Spider-Man or Superman.
In the first X-Men, the villain is Magneto (Ian McKellen), a mutant ideologue and former ally of Professor X (Patrick Stewart), and his plan involves using a crazy machine to make all humans mutants (he doesn't know it mostly just turns humans into goo). He's accompanied by a batch of goons (Mystique, Sabretooth and Toad) but his motivations, rooted in his experience as a Holocaust survivor, are complex. Magneto is involved in every sequel and does lots of bad things, but his lasting relationship with Xavier is always there to remind us that he represents more of a philosophical extreme than anything.
X2 and X-Men Origins: Wolverine include William Stryker, who experiments on mutants like Wolverine to give him metal bones, and X-Men: The Last Stand included the threat of Phoenix (Famke Janssen), an overpowered Jean Grey corrupted by darkness. X-Men: First Class saw Kevin Bacon hamming it up as Sebastian Shaw, but he was once again more of a distraction in the battle of wills between Magneto and Xavier. Days of Future Past had the best kind of sub-villain in the Sentinels — robotic, unambiguously scary mutant-hunter, that exist entirely without personality. But even Peter Dinklage's Bolivar Trask was not the cackling mustache-twirler he could have been, presenting his vendetta against mutants as one he thought could unite humankind as a planet.