Saw this in mid-90s on channel four (UK).
Probably made in mid-70s
There’s a scene, possible opening or closing scene with some sort of occult ritual, people walking down stone steps in a mansion house/castle basement (possibly hidden section of the house).
There’s a couple who are staying at this house/castle and the man is seduced by a lady in front of an open fireplace, she’s on top of the man and is wearing a cross or some necklace. This was taking place while his wife was sleeping or was ill in bed.
The man’s wife is sacrificed in the end some cult. She possibly looked like a dead person from a painting!
It’s so long ago since I saw it, I’m afraid I may be mixing two films up.