Crypt of the Vampire (1964)

"How much shock can the human brain endure before it cracks?"

Count Karnstein sends for a doctor to help his sick daughter Laura. Her nurse believes she is possessed by the spirit of a dead ancestor, Carmilla. A young woman becomes intrigued by the mysterious deaths surrounding Laura after a carriage accident outside the castle forces her to stay. They become close friends until Laura becomes convinced the spirit of Carmilla is forcing her to kill.