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.Of course, what exactly in the world is Yog-Sothoth? Known by various names or nicknames, including The Lurker at the Threshold, The Eater of Souls, 'Umr at-Tawil, and others. Yog-Sothoth is considered by many to merely be a literary personification of the idea of using sorcery to travel between worlds to the Realms Beyond, both the somewhat more earthlike Near Realms, and the considerably more alien Far Realms.
One of the strangest cases, the details of which were purged and surpressed by Terassan authorities, but which are known to a few daring students of black esoterica, is that of the small village of Dunvicus. The Green Book, which can be found in a few mouldering collections, tells of one Lavinia Vatleigh, a balshatoi woman who lived deep in the marshy hills outside of Dunvicus with her father, Old Man Vatleigh. Officially, her father was accused of incest when Lavinia became pregnant and gave birth to a strange boy, Vilbur, although the Old Man always denied it. Yog-Sothoth himself was the father. Vilbur was a strange child, who grew quickly and seemed intelligent after a fashion, but who always inspired loathing and dread where ever he want. Vilbur disappears from the public record and the Green Book makes no mention of his eventual fate, although whatever sinister design Old Man Vatleigh had planned seems to have been thwarted, at least for the time being. Shortly after Vilbur's disappearance, the Green Book makes reference to a garbled account of some kind of menace that nearly destroyed the entire village of Dunvicus entirely, and even today, several generations after the event, locals are suspicious and recalcitrant to speak with anyone who mentions the Unpleasantness, as they call it. The Green Book suggests, horrifyingly, that Vilbur had a twin brother--one who resembled more closely his unearthly father, Yog-Sothoth, rather than his earthly mother Lavinia.
Otherwise, however, references to Yog-Sothoth are vague and sporadic. The name is known to students of the occult, but much of his or its nature is not, nor is their agreement on who or what it is, if it is indeed a thing at all, and not just a cosmic principle like gravity, for instance.
The Green Book does make reference to Yog-Sothoth as a being of blasphemous glowing orbs and ever-changing shape. Few take this literally.