Tsathoggua (nickname: the Sleeper of N'kai) is a fictional character, a Great Old One, a godlike being from the pantheon of H. P. Lovecraft's Cthulhu Mythos, created by Clark Ashton Smith.

"This was a squat, plain temple of basalt blocks without a single carving, and containing only a vacant onyx pedestal.... It has been built in imitation of certain temples depicted in the vaults of Zin, to house a very terrible black toad-idol found in the red-litten world and called Tsathoggua in the Yothic manuscripts. It had been a potent and widely worshipped god, and after its adoption by the people of K’n-yan had lent its name to the city which was later to become dominant in that region. Yothic legend said that it had come from a mysterious inner realm beneath the red-litten world – a black realm of peculiar-sensed beings which had no light at all, but which had had great civilisations and mighty gods before ever the reptilian quadrupeds of Yoth had come into being." -- H.P. Lovecraft, The Mound

"They’ve been inside the earth, too – there are openings which human beings know nothing of – some of them are in these very Vermont hills – and great worlds of unknown life down there; blue-litten K’n-yan, red-litten Yoth, and black, lightless N’kai. It’s from N’kai that frightful Tsathoggua came – you know, the amorphous, toad-like god-creature mentioned in the Pnakotic Manuscripts and the Necronomicon and the Commoriom myth-cycle preserved by the Atlantean high-priest Klarkash-Ton." -- H.P. Lovecraft, The Whisperer in Darkness

"Black Tsathoggua moulded itself from a toad-like gargoyle to a sinuous line with hundreds of rudimentary feet..." -- H.P. Lovecraft, The Horror in the Museum

He dwells in N'kai , where he is to be found asleep. If awakened, he will usually eat the awakener or sacrifice given, then fall back into hibernation. However, there are exceptions: When he was offered the Hyperborean Lord Ralibar Vooz in Smith's "The Seven Geases", he refused him and bound him to a geas to leave and be eaten by another Great Old One (The next six said the same). He is incredibly lazy, refusing to leave his chambers unless it would otherwise prove fatal. Tsathoggua was used extensively in the Cthulhu Mythos work of Robert E. Howard, featuring in stories such as "The Thing on the Roof", "The Black Stone", and "Fire of Asshurbanipal." In these stories he appears as a great toad-like being with elephantine legs, a bloated body that can shoot out tentacles and eyes that destroy the minds of weak willed beings. He is the spawn of Ghizguth and Zystulzhemgni, mate of Shathak, and parent of Zvilpogghua and the Formless Spawn .