The Loreley's Grasp-Helga Liné, Silvia Tortosa-1973 IMDB 5.7/10

The legendary Lorelei has been living for centuries in a grotto beneath the Rhine river 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 Sirgurd is engaged to kill the monster.