The short story “The Landlady” by Roald Dahl is an example of horror fiction.
The aim of horror stories is to frighten or shock readers by describing or suggesting terrifying events. Suspense plays an important part in such stories. In the case of this story, the suspense derives not from the reaction of the main character (who fails to realize what is happening to him), but from the clues which the reader is able to figure out from the text. In a way, the story relies on the readers’ own imagination to scare them.
Horror stories often contain supernatural elements. In this case, Billy’s decision to enter the house despite having decided to go to the Bell and Dragon first is unexplained, almost magical (ll. 100-112).