Theatre of Death (AKA Blood Fiend) is a 1967 British horror movie about a Parisian theater troupe that finds themselves at the center of a spate of mysterious and gruesome murders. The film starred horror icon Christopher Lee, along with Leila Goldoni, Jenny Till, and Julian Glover.
Phillipe Darvas (Lee) is the director at a French theater known as the Theatre de Morte (Theater of Death) which specializes in staging Grand Guignol-style scenes of murder and horror. When new actress Nicole (Till) expresses a reluctance to act out a scene in which she is supposed to attack a helpless victim with a burning fireplace poker, Darvas hypnotizes her into carrying out the scene. Nicole is so deeply hypnotized that she nearly burns fellow actress Dani (Goldoni) for real.
Meanwhile, blood-drained corpses are turning up around the city. The police suspect Darvas, but he disappears. Dani and Charles (Glover) suspect Nicole, but their efforts to follow her around turn up nothing. However, a chance encounter in a Romanian restaurant lead Charles and Dani to discover that Nicole was trapped in an avalanche as a baby, and survived only because her mother fed her human blood. They correctly theorize that Nicole is murdering people in Paris to drink their blood.
Nicole corners Dani and uses hypnosis to make Dani confess, in writing, to all of the murders. She then orders Dani to kill herself, but luckily Charles arrives in time to save her. Nicole flees back to the Theater, where she is accidentally killed during the latest bloody stage show.