One of my favorite underrated films is The Ninth Configuration, directed and written by William Peter Blatty. It starred Stacy Leach and Scott (The Walking Dead) Wilson, and I previously wrote about it for my Neglected Cinema series. Someone has uploaded the whole movie to YouTube, and its a great way to spend an hour and a half.
Here it is:
https://www.youtube.com/watch?v=sZCcjzOeXlM