Title: Diary of a Madman
Simon Cordier is a well-respected magistrate who visits a condemned prisoner, Louis Girot, just before the man's execution. Girot again pleads his innocence insisting that he has been taken over by a spirit that forced him to commit his crimes. Cordier doesn't believe him and the man suddenly dies. Cordier does however note a rapid change in his personality during their short interview. In the following days, Cordier must face a number of strange occurrences in his home. He begins to wonder if he is sleepwalking but is soon hearing voices and begins to wonder about his sanity. It's recommended to him that he take up sculpting, something he once had an interest in. He develops a relationship with Odette, a gold digger married to a struggling artist, but the evil, invisible spirit soon drives him to murder.
Guy de Maupassant (stories), Robert E. Kent