By Robert Bloch (1917-94).
First appearance: Weird Tales, June 1935.
Reprintings list (HERE).
Short short short story (4 pages).
Online at SFFAudio (HERE) (PDF) and Archive.org (HERE).
"A short story of dual personality"
If your other, hidden self kills you, is it suicide or murder?
Comment: Clearly inspired by Stevenson and Poe, the author gets to revel in Lovecraftian purple prose—much more effectively than his preceptor, we think.
Charming phrases: "worm-demolished scrolls of primal dread"; "the sable silence of the locked room"; "tentacles of terror"; "The reflection from the surface of the knife stabbed through his retina like the fiery ray of a burning sun."
- Being a member in good standing with the Lovecraft circle (HERE), (HERE), and (HERE), Robert Bloch couldn't resist introducing "the 'Mad Arab' Abdul Alhazred's" Necronomicon (HERE) into our story.
HERE), (HERE), and (HERE).
The bottom line: