«Serapion» by Francis Stevens

Serapion is a science fiction horror novel by Francis Stevens (pseudonym for Gertrude B. Bennett), "The woman who invented dark fantasy".
This is one of the most intense, complete and unrelenting tales of psychological horror ever put together. No gore, guts and physical putrescence so common to horror, but the utter dissolution of a human spirit, as told by the victim.
Sophisticated, profound, tightly written, carefully composed, singular in its purpose, and inexorable in its movement towards a powerful conclusion, Serapion is considered one of Francis Stevens' highest achievements as a writer.
Clayton Barbour, the narrator, is a protected bourgeois son just weak enough to allow himself to be overwhelmed by a sly, dissembling force of evil, just strong enough to be constantly tormented by his weakness. Invited by a casual acquaintance, Moore, who sees in him a psychic force, he becomes the inadvertent victim of Moore's wife's contact with a channeled malignant force.
During a séance at the Moores' house, Barbour becomes aware of what he calls a "Fifth Presence," an entity soon known to him by the name Serapion. The nature of Serapion is uncertain. His presence distresses Barbour, but he also wishes–or says he wishes–Barbour to be happy and well loved. To that end–ultimately his own ends–Serapion begins to manipulate events in the lives of Barbour and his associates. Every one of them meets a tragic end, only…
Serapion was first published as a four-part serial in The Argosy from June 19 to July 10, 1920.
Gertrude Barrows Bennett (1883-1948) was the first major female writer of fantasy and science fiction in the United States, publishing her stories under the pseudonym Francis Stevens. Bennett wrote a number of highly acclaimed fantasies between 1917 and 1923 and has been called "the woman who invented dark fantasy".
Her most famous books include Claimed! (which Augustus T. Swift, in a letter to The Argosy called "One of the strangest and most compelling science fantasy novels you will ever read") and the lost world novel The Citadel of Fear.
Bennett has been credited as having "the best claim at creating the new genre of dark fantasy". It has been said that Bennett's writings influenced both H. P. Lovecraft and A. Merritt, both of whom "emulated Bennett's earlier style and themes".