I think this movie would likely be late 70s or early 80s. As I recall the Archangel Gabriel has come down to earth in the form of a human (?) in order to stop Satan (or his child, or the Antichrist) who, now college age, is just reaching the age of self-awareness. The film culminates at a college party that is taking place on island with a big old (castle-like?) house. The teens on the island are being slaughtered by creatures (demons? fallen angels?) who are there to usher in the rise of Satan in human form. If I'm remembering correctly, it has kind of a downer ending as the archangel is not successful in his task. I believe it was played as a straight horror...with a bit of gore and, of course, some nudity thrown in (it ended at a college party, after all).