John Carpenter's Prince of Darkness!

[Hanglyman]

Well, in explanation of what I said, I found it to be slower-paced than his other films, some of the deaths and special effects seemed more comical than horrific, the characters were pretty shallow and unlikeable so it was hard to feel anything when they got killed, and Satan inconsistently alternates between being a dark, shadowy, unfathomable force and taunting them in a very human, almost childish manner. Probably more than anything, the religious angle just didn't do it for me... the sci-fi alien horror in The Thing and the Lovecraftian story of In the Mouth of Madness were much more compelling to me (although I'll admit, the latter was pretty funny as well).

But don't get me wrong, it was a good movie. A lot scarier than your typical horror movie. In particular, the shots of the other side of the mirror really set my imagination going, thinking of Hell not as a place of fire and brimstone, but just a lonely, lifeless, pitch-dark void. Kelly and Catherine getting banished into that place forever really was scary... even though, as RoG pointed out, Catherine didn't even try to save herself.