The Segua (1984)

Encarnacion is a beautiful villager and Petronila is her jealous, evil nemesis. When Petronila's love interest, a fetching young Spanish lieutenant, starts to fall for Encarnacion, Petronila gets even by simply destroying his sanity with a vile potion. Petronila begins to delude Encarnacion with suggestions that she is actually a "segua" or a female spirit that attacks men.