To the Devil a Daughter (1976)

Occult writer John Verney (Widmark) is asked by Henry Beddows (Elliot) to pick up his daughter Catherine (Kinski) from the airport. Catherine is a nun with The Children of the Lord, a mysterious Heretic order located in Bavaria and is allowed to come to visit once a year on her birthday. But after Catherine arrives, Beddows then insists that she stay with Verney. But the order, under the sinister excommunicated heretic Father Michael Rayner (Lee), makes all efforts to get Catherine back and uses black magic to stop Verney as he protects her. Verney learns that the order really harbours a group of practicing Satanists who have prepared Catherine to become an avatar of Astaroth upon her eighteenth birthday.