Apocalypse is weird. He's arguably the X-Men's most powerful foe, but he's not the most famous; he has a generic name, hard-to-defined powers, and his motives change a lot. And now he's also going to be the big bad of the next X-Men film! Here's a helpful primer before you see Apocalypse in cinemas.
1) He's old as heck.
The boy who would take the name Apocalypse was born 5,000 years ago in an area of the Middle East called Aqaba (in modern Jordan). He was born his grey skin and his weird blue lips, which led the superstitious villagers to abandon the baby in the desert. He was found by a tribe of raiders called the Sandstormers, who gave him the name En Sabah Nur -- "The First One." He may even be the world's first mutant.
2) His main goal is making sure only the strongest survive.
Although his name makes him sound like he wants to end the world (or at least kill everyone on it), Apocalypse is really much more interested in evolution -- specifically, making sure the strong survive and the weak die. As a mutant himself, he believes mutants are superior to humans much like Magneto does, but Apocalypse also really wants to create a world where everyone is fighting all the time so the strong are forced to get stronger and the weak get killed off. As such, he's into putting humans in death camps and forcing mutants to fight each other, and he figures he's doing everyone a really big favour by doing this. Of course, he also thinks his job would be easier if he had supreme power and/or ruled the world, so he spends a fair bit of his time trying to achieve that, too.
3) He's one of the most powerful mutants ever.
Besides having extra-super super-strength, Apocalypse can alter his body to suit any need. He can change his mass, size and weight, allowing him to grow the size of the Statue of Liberty. He can turn his limbs into weapons ranging from swords to laser cannons, he can pop out extra arms, he can sprout wings and grow gills… and he can disguise himself as pretty much anyone. It's like he can evolve himself to survive any situation. (Get it?) He has a crazy effective healing power, which makes him almost invulnerable. He can fire energy blasts out of pretty much any part of his body. He can fly, he has super-speed, he has both telepathy and telekinesis. It should not be a surprise that he's held his own against an entire team of X-Men on numerous occasions.
4) He's not just a mutant, though.
When Apocalypse was still a young En Sabah Nur, he was taken to a cave containing devices left by the Celestials, who are a race of super-powerful beings who are basically the gods of the Marvel universe (and not like Thor and Loki are "gods" -- these guys created life, death, and the multiverse). Not only did the technology give him Celestial armour, it gave him new mutant powers while augmenting the rest. Not only does Apocalypse get more powerful as he gets older, he's pretty sure he has plenty of powers he doesn't even know about yet. The only downside to his Celestial improvement is that his body can barely handle it, forcing him to switch human hosts regularly (although his tech makes transferring his consciousness and powers pretty simple). Also, he needs to recharge his insane powers periodically by taking years-long naps, which is pretty much the only respite the X-Men get from him.
5) He has a fondness for selecting four minions and dubbing them Horsemen.
Hey, you know that whole "Four Horsemen of the Apocalypse" part of the book of Revelations in the Bible? As the most recent X-Men: Apocalypse movie trailer pointed out, it was inspired by the villain. He's been taking four powerful minions and dubbing them Death, War, Famine and Pestilence for his entire career as a supervillain. He gives them special powers and, in the chance they don't want to serve him willingly anyway, enslaves their minds. Famous Marvel heroes who have become one of Apocalypse's Horsemen include the Hulk, Wolverine, Gambit, the Sentry, and more (especially in alternate Marvel universes). In the upcoming movie, it appears Apocalypse's riders will be Angel, Storm, Magneto and Psylocke.
6) He has a very intense relationship with the Summers family.
My apologies, because this is the point where the wrecking ball that is X-Men comics continuity comes in. In the modern Marvel universe, Apocalypse first woke up -- about a century earlier than planned -- when he felt the arrival of a strange energy. That energy was the hero Cable, who had travelled back in time to stop Apocalypse from conquering the planet and ruling it for the next 19 centuries. Cable is the Nathan, the son of the X-Men Cyclops and Madeline Pryor, who was secretly a clone of Jean Grey; Nathan was infected with a techno-virus as a baby -- by Apocalypse -- and sent to the future to cure him, where he ended up growing up under Apocalypse's nightmare regime. Cable hates Apocalypse. In the future, someone cloned Cable and Apocalypse stole that baby, raised him, and he ended up the as the insane X-Men villain Stryfe, who hated both Cyclops and Apocalypse because daddy issues. Whew! And this is legitimately just the beginning of their crazy, messed-up relationship.
7) He'll fight anybody, including other villains.
Apocalypse is not much for a team-up, and since other villains plans to kill so-and-so or destroy such-and-such would invariably get in the way of his plans to do the same, he's taken on more than a few bad guys, too. He fought Kang the Conqueror back in ancient Egypt. He kicked Dracula's arse a time or two. He's battled both Thor and Loki to standstills. He helped stop Onslaught, although he was mainly doing it to kill Franklin Richards. He held his own against the High Evolutionary, a supervillain with the power to rearrange sub-atomic particles. And the Eternals, who were created by the Celestials and are basically demigods, simply call Apocalypse their "Ancient Nemesis."
8) He conquered the world for a while.
When Professor X's troubled son Legion -- yes, the guy who's about to have his own show on Fox -- decided to travel back in time to assassinate Magneto, he accidentally shoots and kills his dad before he can form the X-Men. As a result, Apocalypse wakes up early and there's no one to stop him, even though Magneto creates and leads the X-Men in Xavier's place. The end result was 1995's Age of Apocalypse storyline, in which Apocalypse has killed most of humanity, enslaved the rest, and only a few groups of mutants resist him. AoA is most notable for some pretty insane changes to the X-Men, including Rogue becoming Magneto's wife, Colossus killing a lot of X-Men including his beloved Kitty Pride, and the existence of Nate Grey, who was created with the DNA of Cyclops and Jean Grey (making him Cable's half-brother twice removed, or something). Anyways, another person from the future has to travel back in time to erase this mess, but this time is happens to be Bishop.
11) He created Mr. Sinister, and it was a huge mistake.
In 1859, Apocalypse used his Celestial tech to turn Victorian scientists Nathanial Essex into the supervillain Mr. Sinister, and has basically regretted it ever since. Sinister was supposed to help the ancient mutant kill all humans, but Sinister thought this plan was dumb and managed to put Apocalypse back into hibernation. Since that point, Sinister has fought Apocalypse many times, impersonated him a few more, and is the guy who cloned Jean Grey and sent her to Cyclops for the express purpose of them having sex and her giving birth to Cable, the biggest thorn in Apocalypse's side. Mr. Sinister happens the next biggest X-Men foe that hasn't appeared in a movie yet, so if there happens to be another movie in the current continuity, he's probably a lock.
12) He is not celibate.
Also, he's apparently not a big fan of birth control, because he's spent 5,000 years sleeping his way around the world, and he has thousands of descendants, most of whom have no idea they're related to the genocidal maniac. He has an entire clan that has done his bidding over the centuries, and they're all related to him.
13) Sometimes he can actually be helpful...
In his desire to create his perfect minions, he's accidentally managed to help the X-Men on several occasions. When Angel had his wings ripped off, Apocalypse gave him his shiny metal ones; after Wolverine had his adamantium ripped off his bones, Apocalypse put it back. He even grew Sunfire's legs back. Obviously, these gifts all came with a price, but the heroes also managed to get out of their deals eventually. I have no idea how healing X-Men jibes with Apocalypse's mantra of "only the strong should survive," and I doubt he's thought much about it.
14) ...and sometimes he is petty as hell.
When Apocalypse was a young lad in ancient Egypt, he fell for a girl who rejected him on account of his grey skin and weird blue lips. En Sabah Nur patiently waited until the girl grew into an old woman, and, once she was in her death bed, visited her specifically to insult her as she lay dying.
15) He was very upset about "House of M."
So remember in "House of M" when Scarlet Witch went crazy, muttered "No more mutants," and then all but 190 or so mutants were depowered? It was a big deal, and Apocalypse did not care for it one bit, because he felt it tipped the scales too far in humanity's favour over mutants, survival-wise. So Apocalypse went to the United Nations and politely requested that they kill 90% of the world's normal human population to even the score. If they didn't, he would release a plague that would kill all of humanity. Apocalypse didn't manage to pull this off, but it's still impressive that he was polite enough to ask first.
16) He was recently a chill little kid.
Actually, he was a few children. He was recently reincarnated as a child, at which point the ostensibly heroic X-Man named Fantomex murdered him. Apocalypse had another son that was hidden from him. And then Fantomex also raised a clone of Apocalypse into a rather pleasant teenager named Evan Sabahnur who he enrolled in the Jean Grey School for Hire Learning, where he used his powers for good. Evan was kidnapped, tortured, given his own pile of Celestial armour, grew into an adult, and flirted with evil a few times, but he's generally stayed good, calling himself Genesis instead, and is currently living in Deadpool's apartment in Queens.
17) He's being played by Poe Dameron.
In case you didn't know who Oscar Isaac was before he stole your heart in The Force Awakens, that is indeed the same dashing Resistance pilot playing the ancient X-Men antagonist in X-Men: Apocalypse (or possibly he's merely cosplaying as Ivan Ooze). Isaac is very excited about playing the villain, but if you've seen the beloved X-Men cartoon of the '90s and remember Apocalypse's appearances on it, you know Isaac has a lot to live up to: