Love Letters of a Portuguese Nun (1977)

16-year-old Maria is forced into Serra D'Aires convent, secretly run by Satanists. Her confessor is in collusion with the Mother Superior. Maria is tortured, forced into sex with men, women, and the horned Devil, and told that it's all a bad dream. She writes a letter to God, and a Knight rescues her, only to fall into the hands of the Inquisition, put on the rack, and condemned to death like Joan of Arc.