The Horla is a short horror story by Guy de Maupassant. The narrator is a wealthy, single man who suddenly starts feeling sick and is convinced that an evil invisible being is causing this and is tormenting him. In the end the narrator is driven so insane that he torches his house along with his servants (unintentionally), as the fabric between real and imaginary is drawn too thin.
The sanity of the narrator has been questioned and so is his physical health; is it some unknown entity tormenting him or is the narrator just suffering from delusion or psychosis? The narrator finds that his bedside water is always gone by morning, so he tries an experiment and covers his lips with pencil powder and covers the glass bottle with white muslin. In the morning, he finds the muslin untouched but all the water gone. Similar other incidents also take place. Only the narrator seems affected in the house; all his loyal servants are still fine. If the reader believes the narrator, one is forced to believe in the unknown. However, it is also possible that he's suffering from a delusion and imagining all these things. The reader is left to ponder the ending as either the result of an insane person or of a sane person under the influence of the unknown.