Curse of the Crimson Altar (aka The Crimson Cult)

This is a British horror film starring Christopher Lee and Boris Karloff. Antiques dealer Robert Manning is searching for his brother. He stays at a remote house where he is invited by the niece of the owner. Robert has strange dreams of ritual sacrifice. He is assured that his brother is not there, but his suspicions are aroused by a witchcraft cult which is still active in the area. Robert has nightmarish hallucinations about the cult, and the search for his brother will mystify you.