Tragic Ceremony(1972)

Original title: Estratto dagli archivi segreti della polizia di una capitale europea

A foursome of young friends, enjoying themselves in the countryside, happen upon a secluded villa. Believing the place to be empty, they decide to investigate, only to stumble upon a grisly, in progress satanic rite. Barely escaping with their lives, it's not long before evil forces begin to torment them...