Watch Diary of a Madman

After Simon Cordier dies, his diary is read to a large audience. The diary mentions Simon's interactions with a fiendish entity, the horla, which was capable of psychic powers. Simon's interactions with the creature began when a possessed inmate sought to kill him. After Simon kills the inmate, the horla turns its menace toward him.

The horla deludes Simon into believing that his sanity is quickly slipping, plaguing him with second guesses. This continues until Simon believes that a creature is responsible, luring the horla into a fire trap that results in the scenes at the start of the film.