The Toy Box 1971 ( 4.1)

When Ralph persuades his girlfriend, Donna, to participate in a bizarre party, she discovers the guests are to perform erotic plays in the presence of a corpse - Ralph's millionaire uncle. The players are promised gifts from the "toy box." Ralph and Donna await their turn, when the mansion's doors lock at midnight; and it's not long before they discover the players are being murdered by...the dead uncle?