The Shout (1978)

Bored while officiating a cricket match at a psychiatric hospital, Crossley tells Graves (a visitor) the tale of a mysterious stranger (also named Crossley) who invades the lives and house of a local musician and his wife. The stranger claims knowledge of real magic, which he uses to displace his host and dominate his wife. The musician must find a way to combat Crossley and his seemingly implacable powers. Graves doubts Crossley's claim that the story is true, and begins to believe that Crossley is actually one of the patients.