Possessed miners on Mars

First Appearance: John Carpenter's Ghosts of Mars (2001)

A couple of police officers on Mars need to transport a criminal to a remote mining settlement. When they get there it turns out all the miners have been possessed by Martian ghosts. A siege ensues.
This movie is as close as you can get to being a Western as possible with out actually having it set in the old west, using horses or having the heroes fight "injuns." Still wondering when John Carpenter's just going to make that Western he seems to want to make.
Also of note it features a score by John Carpenter (on synthesizer) and a bunch of heavy metal guitarits, including Robin Finck, Steve Vai and Buckethead, and the rest of Anthrax.

Satan

First appearance: Prince of Darkness (1987)

In the bowels of a small run down church a secret is hidden. A group of physics students spend the weekend trying to discover what that secret is. Turns out its a big can of liquid Satan and Jesus was an alien. Can the students stop the awakening evil from bringing Anti-god across the dimensional plane and destroying the universe?

The second movie in John Carpenter's Apocalypse Trilogy (the other two are The Thing and In the Mouth of Madness)

Regan McNeil / Captain Howdy / Pazuzu

First appearance: The Exorcist (1973)

A young girl gets a little to close to her imaginary friend making her actress mom's life miserable. Fortunately there's an old priest and a young priest that know what to do about it.