If you've seen 'Black Sabbath' by Bava, you saw the short 'Wurdulak' which is also based on the same story as this. This is more in the giallo tradition with the nudity, and the blood and gore. It's not as well acted and the framing device of the mental hospital doesn't work that well.
I'm surprised none of Mubi's horror denizens have commented on this eerie little film. Based on Tolstoy's novella The Family of Vourdalak, an urbanite's car breaks down in the Yugoslavian wilderness. The traveler soon stumbles upon a remote cottage occupied by a withdrawn family that guards a dark secret. "Devils" isn't nail-biting terror, but it's a well-paced, creepy lo-fi gem that commands better recognition.