Justin Geoffrey is a Baudelairean poet and author of the terrible People of the Monolith which was completed just before his confinement in an asylum. Each author of the Mythos seems to have created his own horrific work by a fictional author, usually a madman. Geoffrey is Robert E. Howard's version of Lovecraft's Abdul Alhazred as Eibon was Smith's.
From Lin Carter, Lovecraft: A Look Behind the Mythos: "...in 'The Black Stone' Howard invents a "mad poet" of his own, a sort of English-speaking version of Abdul Alhazred named Justin Geoffrey, author of a nightmarish poem entitled "The People of the Monolith." A versifier of considerable gifts, Howard gives us a morsel of Geoffrey's talent by quoting a few lines of his as the heading of this story:They say foul beings of Old Times still lurkAnother story, 'The Thing on the Roof,' picks up and elaborates on both Justin Geoffrey and Von Junzt, and introduces Xuthltan, who plays a major part in yet another tale, 'The Fire of Asshurbanipal,' not to be published for some years."
In dark forgotten corners of the world,
And Gates still gape to loose, on certain nights,
Shape pent in Hell.
[The Thing on the Doorstep - H.P.L.]
See also: People of the Monolith