The Case of Charles Dexter Ward
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.