Known also as The Curse of the Crimson Altar and just The Crimson Altar, The Crimson Cult (as it was called stateside) features the final filmed role of Boris Karloff (although other films would be released posthumously). The film also stars Christopher Lee, Barbara Steele and Michael Gough. Tigon was the production company that made this, trying to do battle with Hammer and also Amicus in the late 60s and early 70s. You would be forgiven for thinking this was meant to be an anthology film, seeing as how the three main leads have their performances all over the place (and not together in any sense of the word) and how the thing feels weird and disconnected...even for 1968. But no, it's based on Lovecraft's "Dreams in the Witch House"--though no credit is given.
And Barbara Steele's Lavinia voice is so godawful with all the effects they put on it, it makes you beg for a vocoder...which ordinarily people will open their veins to avoid. For the film, I recommend a pairing with an alcoholic beverage. And I recommend...anything with alcohol in it. Enjoy.
The film is out of print on Region 1 and only now coming back into print in Europe. Go figure.