Satan's blood (1978)

After a chance encounter with a mysterious couple claiming to be old friends, Andrés and Ana are invited to spend the evening at a beautiful, secluded old villa. As the night winds down, they begin to sense that there are some rather strange things going on around them and, after agreeing to sleep over, find themselves unwittingly pulled into a series of bizarre sexual encounters which they slowly discover are part of a horrifying Satanic ritual designed to make them slaves to the Prince of Darkness!