Horla is a long fantastic and psychological novella, which mixes the supernatural with the natural. It is considered to be the first fantasy story that describes the mental evolution of a psychiatric disorder. He himself was suffering from a mental disorder at the time, which led to his death 5 years later.
The narrator tells us about his visions, his thoughts and what he feels through his five senses. he believes that he is being pursued by an invisible creature he calls Horla, which makes him go through several psychic states that torture him, which he tells us about.
Paranoia, hallucinations, anxiety attacks, sleep paralysis are all states he goes through before sinking into dementia.
The book describes each stage in such detail that the reader ends up believing in it and no longer discerning what is true and what is false. Indeed, Maupassant creates a strong connection between the reader and the character who speaks in the first person.