Ythogtha, also known as The Thing in the Pit, is the second son of Cthulhu and one of the Ogdru Hem. Ythogtha was spawned by Cthulhu and Idh-yaa on the distant planet Xoth, Ythogtha was 'brother' to Ghatanothoa, Zoth-Ommog and Cthylla.
Ythogtha is incredibly large; when the sorcerer-priest Zanthu attempts to free Ythogtha from his prison, he mistakes his god's clawed, slimy fingertips for mountainous heads. It resembles a gigantic, humanoid frog-like appearance with only a single, huge eye in the center of his forehead like a cyclops. A dense mane and a beard of writhing tentacles grows from his head.
Ythogtha was one of the many Ogdru Hem imprisoned by the gods after the defeat of his father Cthulhu. Ythogtha was contained within the Abyss of Yhe, and was worshipped as a god by both the Yugg and various tribes of hominids who inhabited the lost continent of Mu. He is also served by the lord of the Yugg known as Ubb, the Father of Worms.
After the demise of the high priest of Shub-Niggurath at the hands of Ghatanothoa, the Cult of Ghatanothoa became the dominant religion on Mu, and its priesthood outlawed all other religions. To overthrow the Cult of Ghatanothoa and restore the declining Cult of Ythogtha to its rightful place as chief religion of Mu, the high priest Zanthu attempted to free his master Ythogtha.
But once he saw the true nature of this beast, abandoned the ritual and fled Mu, recording his endeavors on the twelve "Zanthu Tablets," which also contained all of Ythogtha's sacred rituals and secrets. Moreover, Zanthu was the one who alerted the people of Agartha in regards to this unspeakable horror that would encompass all of Mu which resulted in a battle between Ghatanothoa and Agartha leading to the latter's victory over the Old One. Ythogtha, however, still remained and would seize this opportunity to place himself in the center of Mu's worship.
Fortunately, this did not come to be, as the Elohim were displeased with the people of Mu but answered the request from Agartha. Kanaloa and Poseidon destroyed the continent of Mu, sending it to the bottom of the ocean along with Ythogtha himself.