Possession is a 1981 West German film from director/writer Andrzej Zulawski featuring Sam Neill (of Jurassic Park fame) and Isabelle Adjani, who won Best Actress at Cannes for her performance---and rightly so. She is stunning.
The most important thing I learned from this film: Do not go off to be some Cold War spy and let your wife get possessed by some nasty demon. It is not pretty.
I won't tell you too much about what happens, but Possession is kind of like Rosemary's Baby meets Antichrist plus Invasion of the Body Snatchers with a Euro-arthouse slant. That's right.
We are not getting along.
Do not get the kitchen staff involved in your argument. You will not win.
New Age mystics are not to be trusted.
Isabelle is a knockout.
She can also do crazy.
This is how I learned about Possession. (The blog post was about Wages of Fear, a film I have not seen but will some day.)
Possession is out of print in the US right now (like a lot of stuff we've been watching in movie club lately) but imports are available.