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.

Source: TMDb