Possession is a very weird movie. It's about a spy who returns home to his wife who wants a divorce because she's been seeing another man. However, the spy soon learns it's not a man at all.
If you asked me what Possession is about, that's what I'd tell you. However, I would describe Possession as an experience. Every second of this movie is a surreal experience that leaves you questioning your sanity.
Everything from the cinematography to the soundtrack leaves you feeling more and more uneasy with the ride you're on. If you're expecting a straight-forward narrative about a man finding out his wife is fucking a demon, you'll be disappointed.
However, if you go into this expecting a mind fuck of a movie that leaves an impression upon you long after it's over, I highly recommend Possession.