Demonic Man Turns into a Woman

I remember seeing a movie on HBO or Cinemax when I was a kid in the late 70's early 80's.

Most of what I remember is about this guy that gets jumped in the shower by these other guys at a college or something. They don't know that he actually is possessed or is the devil himself and he gets revenge on them. In one part of the movie he turns into a woman.

There was a castle in the movie and it was in color at the time. There was a moment were there was a big gold cross and the guy gets killed at the end. Very scary movie, would be R rated and had some homo erotic-like undertones. Not that thats the interest, just the movie as it was scary as hell. I have never seen a movie more worth finding if you like horror...

PLEASE SOMEONE HELP ME FIND THIS MOVIE OR THE NAME OF IT?