Simon Cordier, a French magistrate and amateur sculptor comes into contact with a malevolent entity. The invisible - yet corporeal - being, called a "horla" is capable of limited psychokinesis and complete mind control.
Unfortunately the movie Diary of a Madman is not yet available on HBO Max.
| Art | Daniel Haller | Art Direction |
| Writing | Robert E. Kent | Screenplay |
| Production | Robert E. Kent | Producer |
| Directing | Reginald Le Borg | Director |
| Sound | Edna Bullock | Music Editor |
| Sound | Alfred R. Bird | Sound Effects Editor |
| Art | Lowell Grant | Sculptor |
| Crew | Norman Breedlove | Special Effects |
| Art | Irving W. Sindler | Property Master |
| Sound | Ralph Butler | Sound |
| Editing | Grant Whytock | Editor |
| Costume & Make-Up | Marjorie Corso | Costume Design |
| Art | Victor A. Gangelin | Set Decoration |
| Camera | Ellis W. Carter | Director of Photography |
| Sound | Richard LaSalle | Original Music Composer |
| Directing | Jane Ficker | Script Supervisor |
| Writing | Guy de Maupassant | Short Story |
| Production | Joseph Small | Production Manager |
| Directing | Al Westen | Assistant Director |
| Costume & Make-Up | Ted Coodley | Makeup Artist |
| Costume & Make-Up | Carmen Dirigo | Hairstylist |
| Production | Edward Small | Producer |
