Apparently this movie wasn't very well reviewed when it came out. I can kind of understand that - there's nothing in it that especially sets it apart from other occult/possession movies. At the same time, it checks off all of the elements that an occult/possession movie ought to have, and sometimes (in my book, anyway) that's good enough. Let's review.
John Verney is a writer about the occult. An old acquaintance asks him to pick up his daughter, Catherine, from the airport and watch over her. Catherine is a nun in an order called The Children of the Lord, which was founded by a heretic priest. She's been raised in seclusion her entire life, in preparation to become the living embodiment of the demon Astaroth. Verney tries to protect her against the efforts of the evil priest, who uses the nefarious powers under his control to try and get Catherine back.
So, let's look at what we've got here.
Innocent girl in peril from evil forces? Check! (Also, didn't we just talk about that? Yes. Yes we did.)
Sinister secret cult? Check!
Secret tomes of ancient lore? Check!
A dramatic final confrontation between the forces of good and evil? Check!
It covers all the bases, man. Not only that, it stars Nastassja Kinski. That doesn't mean much these days, but to guys of a certain age (read: MY age), that's still kind of a big deal. Also, Christopher Lee is the heretic priest! Christopher Lee is born to play heretic priests! You know what's great about him? His smile. When he smiles, it's even scarier than when he scowls, because it means he's happy about something bad that's about to happen.
So, to recap things that we must take into consideration when judging this movie:
Nastassja Kinski: +1,000,000
Christopher Lee: +1,000,000
Nastassja Kinski gets nekkid: +2,000,000
Christopher Lee also gets nekkid: -3,999,995
That leaves us with a total of five points, which I'm awarding in the form of five Astaroths out of five.