Imagination called up the shocking form of fabulous Yog-Sothoth — only a congeries of iridescent globes, yet stupendous in its malign suggestiveness.
—H. P. Lovecraft, "The Horror in the Museum"
Yog-Sothoth is an Outer God and is coterminous with all time and space yet is supposedly locked outside of the universe we inhabit. Its cosmic nature is hinted at in this passage from "Through the Gates of the Silver Key" (1934) by Lovecraft and E. Hoffmann Price:
It was an All-in-One and One-in-All of limitless being and self — not merely a thing of one Space-Time continuum, but allied to the ultimate animating essence of existence's whole unbounded sweep — the last, utter sweep which has no confines and which outreaches fancy and mathematics alike. It was perhaps that which certain secret cults of earth have whispered of as YOG-SOTHOTH, and which has been a deity under other names; that which the crustaceans of Yuggoth worship as the Beyond-One, and which the vaporous brains of the spiral nebulae know by an untranslatable Sign...
Yog-Sothoth knows all and sees all. To "please" this deity could bring knowledge of many things. However, like most beings in the mythos, to see it or learn too much about it is to court disaster. Some authors state that the favour of the god requires a human sacrifice or eternal servitude.
The essay In Rerum Supernatura in the Call of Cthulhu role-playing game suggests that Yog-Sothoth's name may be a rough transliteration of the Arabic phrase "Yaji Ash-Shuthath," meaning "There is no peace at the gates.
Yog-Sothoth knows the gate. Yog-Sothoth is the gate. Yog-Sothoth is the key and guardian of the gate. Past, present, future, all are one in Yog-Sothoth. He knows where the Old Ones broke through of old, and where They shall break through again. He knows where They have trod earth's fields, and where They still tread them, and why no one can behold Them as They tread.
—H. P. Lovecraft, "The Dunwich Horror"
Yog-Sothoth has some connection to the mysterious Old Ones mentioned in "The Dunwich Horror" (1929), but their nature, their number, and their connection to Yog-Sothoth are unknown. Nonetheless, they are probably allied to him in some way, since Wilbur Whateley, the half-human son of Yog-Sothoth, tried to summon them so that they could control Wilbur's more tainted twin and make it reproduce.
In The Case of Charles Dexter Ward, its name is part of an incantation that could revive the dead:
Aforgomon is an obscure avatar of Yog-Sothoth invented by Clark Ashton Smith. He was revered by many cultures past, present, and future as the God of Time because of his preternatural ability to manipulate time and space. Little is known of this being's appearance because he only reveals himself to those who have angered him. However, it is known that he is accompanied by a blinding light. He is the mortal enemy of Xexanoth.
...great globes of light massing toward the opening, and not alone these, but the breaking apart of the nearest globes, and the protoplasmic flesh that flowed blackly outward to join together and form that eldritch, hideous horror from outer space, that spawn of the blankness of primal time, that tentacled amorphous monster which was the lurker at the threshold, whose mask was as a congeries of iridescent globes, the noxious Yog-Sothoth, who froths as primal slime in nuclear chaos beyond the nethermost outposts of space and time!
'Umr at-Tawil (The Most Ancient and Prolonged of Life), also spelled Tawil At-U'mr or Tawil-at'Umr, is described as an avatar of Yog-Sothoth in the story "Through the Gates of the Silver Key", by Lovecraft and E. Hoffman Price. In the story, he presides over the timeless halls beyond the Gate of the Silver Key and the strange, near-omnipotent Ancient Ones that dwell there. He is described as the silhouette of a man behind a strange, shimmering veil.
Though not as ubiquitous as Cthulhu, Yog-Sothoth sometimes appears in popular culture as a way to evoke Lovecraftian themes.
Yog-Sothoth occasionally appears in fiction outside the Cthulhu Mythos milieu:
Bands in the heavy metal genre frequently make reference to Yog-Sothoth. It is also referred to, in veiled or open form, by a number of video games and television shows.