Tragic Ceremony (1972)

After a peaceful sailboat ride, four young people, including rich kid Bill, Joe, Fred and Jane, knock on the door of a secluded villa after their dune buggy runs out of gas. Earlier in the day, Bill had given the lovely Jane a pearl necklace with a supposedly paranormal history, and this later opens up a can of worms. They are invited to spend the night at the mansion, owned by Lord Alexander and Lady Alexander, who happen to be hosting a strange ceremony that night attended by a group of eccentrics in black robes. During the evening, Jane exits her sleep chamber, seemingly in some kind of trance, and is lured to a sacrificial alter where the robed houseguests are hovering over her. As a knife is about to be plunged into the young lady, her three friends come to the rescue, but they are also witness to a chaotic mass murder catastrophe in which they flee with feelings of guilt and uncertainty.