Horror Upon Horror
Horror is not a self-contained genre. It can, for example, include ghost stories in which the ghost is malevolent or odious, and the classic Gothic Horror where horror can work alongside romance. Also these are vampire novels, others based on supernatural elements of folklore, and another group generally referred to as novels of the mind (psychological stories in which the main character often descends into madness) and many others.
What holds all these together under the umbrella term horror is that their writers aim to create an irrational fear of the unnatural or the supernatural.
The genre has been around a long time: we all know the work of 19th century Edgar Allan Poe but you can trace its history further back than that to include Horace Walpole’s 1764 novel The Castle of Otranto.
Suzanne Ruthven argues that there are common principles that run through each of the sub-genres, and these are explored and explained in her book which focuses on the art and craft of writing horror.