In Guy de Maupassant's "The Horla," the narrator is convinced that some unknown and invisible being has descended upon him (and his house) and is killing him slowly by swaying his mind as a precursor to alien invasion on Earth. The situation worsens as the story progresses, with the narrator burning down his house in the hopes of trapping and killing this entity in the end.
The unknown creature leads to conflicts in narrator's mind. The narrator finds his water bottle empty in the morning, yet he does not remember drinking it. Some milk is also gone, yet honey, strawberries and bread are left alone. The narrator can see these physical changes, yet has no understanding of how this is happening; is it an unknown entity or is he going insane? He can hardly sleep as he feels the malevolent presence in his room. His health improves as he moves away from his house but deteriorates the longer he stays there; without any apparent cause, his doctor cannot seem to help him either. The elements of the supernatural are there for the reader, but so is the question: is it a supernatural invisible being at play or is the narrator going insane and turning psychotic?