SPOILER WARNING: This article discusses the ending and post-credits scene in X-Men: Days of Future Past.
All right, so Days of Future Past was a ton of fun. That ending! Cyclops is alive everyone! But what did that brief and strange post-credits scene mean? Let's break it down.
The scene opens in ancient Egypt, revealing a thin gray boy concentrating on the act of constructing a pyramid with the power of his mind while surrounded by worshippers chanting "En Sabah Nur." As the camera pans over his shoulder, we see four men on horseback, watching from the top of a nearby sand dune.
The gray-skinned boy is Apocalypse (real name En Sabah Nur), the first mutant ever born, who has a combination of pretty much every mutant ability. He's immortal, super strong, super fast, telekinetic, psychic, and capable of shape-shifting to varying degrees (depending on who's doing the writing). And, naturally, the riders atop the sand dune are his Four Horsemen.
If you said, "But hey, no one in ancient Egypt would understand that Biblical allusion," then congratulations, you deserve a candy bar.
The Next Movie
The next movie's title, X-Men: Apocalypse, is a pretty clear indication we'll get to see plenty of this new villain. And like DOFP, we'll probably see some key plot points borrowed from the comics.
Age of Apocalypse is the biggest story involving the powerful mutant. It's a very big, convoluted and operatic storyline that sprawls across several X-titles (X-Calibre, Astonishing X-Men, Generation NeXt, X-Man, Amazing X-Men, and so many more X-puns). It was published during the mid '90s comics boom, and takes place mostly in an alternate timeline where the entire world lies in ruins not unlike what we saw in the future world of DOFP. This world is ruled by Apocalypse, who really really buys into the whole "survival of the fittest" thing. He's a tyrant whose quest for genetic perfection rings with notes of Hitlerian eugenics familiar to X-Men fans.
The comics story pulls together several generations of X-Men characters, and throws the world into so much chaos their good and bad alignments are totally up in the air.
The Four Horsemen
In the comics, Apocalypse likes to keep four henchmen particularly close, assigning them the roles of Plague, Famine, War, and Death. The actual super-powered individuals in those roles rotates, but most famously Apocalypse gave the title of Death to Angel (Warren Worthington III, who was seen in X-Men 3) after brainwashing him and replacing his recently amputated wings with metal cyber wings.
More About Apocalypse
As an immortal who's been around since ancient times, Apocalypse has messed with history all over the place. He traveled to Aztec Mexico to play the part of Mesoamerican war god Huitzilopochtli, and has also taken on the identities of Hindi god of destruction Kali, and Egyptian god of violence Set. He also (not kidding) is responsible for both creating and destroying Dracula.
Only a handful of Marvel characters have been capable of standing up to Apocalypse, among them a super mutant named Cable, whose parents happen to be Cyclops and Jean Grey. Not much is known yet about the next movie, but it wouldn't be surprising to see the fan-favorite character show up in it in some capacity.