Cthulhu Mythos
Although Lovecraft first mentioned "Yog-Sothoth" in the novel The Case of Charles Dexter Ward, it was in "The Dunwich Horror" that he introduced the entity as one of his extra-dimensional Outer Gods. It is also the tale in which the Necronomicon makes the most significant appearance, and the longest direct quote from it appears in the text. Many of the other standards of the Cthulhu Mythos, such as Miskatonic University, Arkham and Dunwich also form integral parts of the tale.
A librarian named Armitage appears in Don Webb's short story "To Mars and Providence", an alternate history where a juvenile Lovecraft is influenced by the events of H.G. Wells's The War of the Worlds.
Read more about this topic: The Dunwich Horror