First Showing has talked to an insider and a second source and has revealed that following
DAYS OF FUTURE PAST the post-credit scene will feature Michael Fassbender’s Magneto discovering Apocalypse and becoming possessed by him. Here is their specific description of the scene:

"...the credits bumper jumps back to the past with a young Erik Lehnsherr (Michael Fassbender) in a ramshackle house in the desert eating dinner, clearly on the run from all the authorities who want to bring him down. While using a well outside to get some water, the young Magneto sees something fall from the night sky and crash to Earth nearby. Upon inspecting the impact zone, he encounters what is described as an "alien form" who is "towering, terrifying, with a twisted metal face and dark pits for eyes." It's Apocalypse and he says, "I have crossed rivers of time to find you." Magneto asks, "Who are you?"

The alien responds, "I am your future." Suddenly Apocalypse merges with Magneto, who screams and blasts an insane amount of energy and shockwaves around him. This energy explosion levels everything around him, with a blast radius that goes for miles. As Magneto looks at what he's done, he has a realization of the power he now holds and a villainous grin begins to form before the film cuts to black."