[2] In the story, Warren is a mysterious occultist and friend of Carter (Lovecraft's alter ego), who suffers a gruesome but undefined fate while exploring a crypt in Big Cypress Swamp.
[3] In "The Silver Key", which he wrote in 1926, he describes Harley Warren as "a man in the south, who was shunned and feared for the blaspemous [sic] things he read in prehistoric books and clay tablets smuggled from India and Arabia".
The story's sequel, "Through the Gates of the Silver Key", presents Warren as an expert linguist who has mastered the primal Naacal language of the Himalayas.
What singles out Warren from Lovecraft's many other doomed characters is his self-sacrificial, gentle nature (he pleads with Carter to put back the crypt's slab and flee while he still has a chance to get away), combined with a sinister element that is more in keeping with Lovecraft's other ambiguous anti-heroes such as Richard Upton Pickman and Herbert West (Carter describes Warren's expression as "disquieting" when he talks about his occult theories).
A character who bears a resemblance to Warren is Clark Ashton Smith's antehuman sorcerer Haon-Dor (from "The Seven Geases"), another seeker after forbidden lore.