The Apocalypse character, created by Louise Simonson and Walter Simonson, started out as a main adversary to the X-Men comic book spin-off X-Factor, before crossing over into the X-Men comics. The character, whose birth name is En Sabah Nur, is actually the first mutant in human history, born in Egypt around the year 3,000 B.C.
Apocalypse possesses a number of powers, such as superhuman strength, teleportation and he can also manipulate energy. While he was born with a number of abilities, his powers were augmented after merging himself with Celestial alien technology.
In a 1995 arc known as Age of the Apocalypse, an alternate timeline was created where the main Marvel universe ceased to exist and Apocalypse ruled over most of the known world. X-Men: Days of Future Past does feature a time travel element to the story, so it seems that could lead into this story line. The 2014 release also includes key figures from the Age of Apocalypse comics, such as Bishop (Omar Sy) and Blink (Bingbing Fan).
Of course, nothing has been confirmed by Marvel at this time, except for the release date and the title.