Serapion

In a career that spanned a mere three years, Gertrude Barrows Bennett (writing as Francis Stevens) published half a dozen books that came to define the genres that followed on. she is most popularly known as the woman who invented dark fantasy, but on the way she also invented a new, creepier kind of dystopian Sci Fi.
Today, you would call this a tale of trauma-onset schizophrenia, or perhaps a terrible descent into Dissociative Identity Disorder, foretold by genetics. In 1920, the best way to make sense of it was demonic possession. A true masterwork of psychological horror, from before psychology existed.
"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. It is also perhaps the saddest book I've ever read, a perfectly realized story of unredeemed personal degradation and its effects on all it touches.
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 to a séance 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.
From this point on, the life of Clayton, his family and his friends is slowly, inextricably ripped asunder by events and in ways that seem unconnected but are manipulated by the Fifth Presence within him. Bennett pulls no punches, provides no happy ending. In that, it is her most honest work (and perhaps a summing summing up of her own life to this point, when she had lost a husband, father and invalid mother)...
Dark, wrenching, truly horrifying, but a book I can recommend without the least reservation." [Derek Davis, Goodreads]