Byleth: The Demon of Incest is a 1972 Italian gothic horror film directed and written by Leopoldo Savona. Set in 19th-century Italy, the film features Mark Damon as Lionello Shandell who welcomes his sister Barbara and her new husband Giordano back to their castle after their trip to England. The two siblings are strongly affectionate for each other and the new marriage has caused Lionello into a mad jealousy which Giordano tries to cure as murders begin mysteriously happening around the castle.