Las garras de Lorelei (1976)

Director: Amando de Ossorio
Cast: Francisco Nieto, Helga Liné, José Thelman, Josefina Jartin, Loreta Tovar, Luis Induni, Silvia Tortosa, Tony Kendall

Synopsis:

The legendary Loreley has been living for centuries in a grotto beneath the river Rhein in Germany. Every night when the moon is full, she turns into a reptile-like creature craving for human blood. When one girl after another of a nearby boarding school is killed by her, a hunter named Sigurd is engaged to kill the monster.