Byleth: The Demon of Incest (Italian: Byleth (il demone dell'incesto)) 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 (Claudia Gravy) and her new husband Giordano (Aldo Bufi Landi) 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.

Byleth: The Demon of Incest was shot in Rome and released in Italy and later in Germany where it was re-edited and featured more explicit scenes of nudity and different editing.