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.

“This is the last novel published by Gertrude Barrows Bennett, using throughout her brief career the pen name of Francis Stevens. Like her others, it ran as a serialization, this time in the leading fantasy magazine of its day, Argosy. All her serializations appeared between 1918 and 1920.
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. 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 s√©ance 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 up of her own life to this point, when she had lost a husband, father and invalid mother). So far as the published record goes, she wrote nothing more. And after Serapion, it’s hard to imagine what more she could write.
Dark, wrenching, truly horrifying, but a book I can recommend without the least reservation.” [derek davis, goodreads]

Book versions Illustrated with images by Virgil Finlay, from the original editions of Stevens’ work.