Tsathoggua (or Zhothaqquah) is described as an Old One, a godlike being from the pantheon. He was invented in Smith's short story "The Tale of Satampra Zeiros," written in 1929 and published in the November 1931 issue of Weird Tales. His first appearance in print, however, was in H. P. Lovecraft's story "The Whisperer in Darkness", written in 1930 and published in the August 1931 Weird Tales.
The first description of Tsathoggua occurs in "The Tale of Satampra Zeiros", in which the protagonists encounter one of the entity's idols:
Later, in Smith's "The Seven Geases" (1933), Tsathoggua is described again:
[In] that secret cave in the bowels of Voormithadreth . . . abides from eldermost eons the god Tsathoggua. You shall know Tsathoggua by his great girth and his batlike furriness and the look of a sleepy black toad which he has eternally. He will rise not from his place, even in the ravening of hunger, but will wait in divine slothfulness for the sacrifice.
—Clark Ashton Smith, "The Seven Geases"
Robert M. Price notes that "Lovecraft's Tsathoggua and Smith's differ at practically every point." Lovecraft, dropping Smith's bat and sloth comparisons, refers to the entity in "The Whisperer in Darkness" as 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--the priest's name a tip of the hat to Tsathoggua's creator.
Later, in "The Horror in the Museum", a story ghost-written by Lovecraft, he writes, "Black Tsathoggua moulded itself from a toad-like gargoyle to a sinuous line with hundreds of rudimentary feet."
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 and Zealia Bishop, "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"
Tsathoggua dwells deep beneath the earth in N'kai. Tsathoggua once dwelt inside Mount Voormithadreth in Hyberborea, but left after the continent iced over.
The basin ... was filled with a sort of viscous and semi-liquescent substance, quite opaque and of a sooty color.... [T]he center swelled as if with the action of some powerful yeast [and] an uncouth amorphous head with dull and bulging eyes arose gradually on an ever-lengthening neck ... Then two arms — if one could call them arms — likewise arose inch by inch, and we saw that the thing was not ... a creature immersed in the liquid, but that the liquid itself had put forth this hideous neck and head, and [it was now forming arms] that groped toward us with tentacle-like appendages in lieu of claws or hands! ... Then the whole mass of the dark fluid began to rise [and] poured over the rim of the basin like a torrent of black quicksilver, taking as it reached the floor an undulant ophidian form which immediately developed more than a dozen short legs.
—Clark Ashton Smith, "The Tale of Satampra Zeiros"
Tsathoggua's will is carried out by the formless spawn, polymorphic entities made of black goo. They are extremely resilient and very difficult to dispatch. Formless spawn can take any shape and can attack their targets in nearly every conceivable way. They are surprisingly flexible and plastic-like, and can quickly flow into a room through the tiniest of cracks. They attack by trampling their targets, biting them, or crushing them with their grasp. The Call of Cthulhu roleplaying game's entry on Formless Spawn also claims that they are powerfully acidic in substance and can dissolve human flesh with even a slight touch.
Formless spawn often rest in basins in Tsathoggua's temples and keep the sanctuary from being defiled by nonbelievers.
In his story At the Mountains of Madness, H. P. Lovecraft states that "[a] few daring mystics have hinted at a pre-Pleistocene origin for the fragmentary Pnakotic Manuscripts, and have suggested that the devotees of Tsathoggua were as alien to mankind as Tsathoggua itself."
Smith literally wed Lovecraft's creations to his own gods, which seem to be molded more like the Greek pantheon than the cosmic group of Lovecraft's fiction. Indeed, he assigned outlandish familial relationships to his gods — for example, making the Saturnian being Hziulquoigmnzhah the "uncle" of Tsathoggua — and ascribed this bizarre family tree to the Parchments of Pnom, Hyperborea's leading "genealogist [ noted prophet".
According to Pnom, Tsathoggua is the spawn of Ghisguth and Zystulzhemgni. He is the mate of Shathak and the parent of Zvilpogghua.
Cxaxukluth (or Ksaksa-Kluth) is a Great Old One and is the "son" of Azathoth by spontaneous fission. His progeny are Hziulquoigmnzhah and Ghisguth. He is the grandfather of Tsathoggua.
Cxaxukluth dwells on Yuggoth. His immediate family lived with him for a while, but soon left because of his cannibalistic appetites.
Ghisguth (or Ghizghuth or Ghisghuth) is the son of Cxaxukluth and the brother of Hziulquoigmnzhah. He is the mate of Zstylzhemghi and the father of Tsathoggua.
Hziulquoigmnzhah (also Ziulquaz-Manzah) is the son of Cxaxukluth. He is also the brother to Ghisguth and the uncle of Tsathoggua.
His appearance is much like his nephew, but he has an elongated neck, very long forelimbs, and very short, multiple legs. He has had many homes including Xoth (possibly Sirius B), Yaksh (Neptune), and Cykranosh (Saturn), where he resides to this day.
Knygathin Zhaum is the child of Sfatlicllp and a Voormi.
He repopulated Hyperborea after humans deserted the cities of Uzuldaroum and Commoriom. Athammaus tried to execute him by beheading, but because of his preternatural heritage, such attempts proved unsuccessful and only served to aggravate him. As a descendant of Cxaxukluth, Knygathin Zhaum reproduced by fission and thus created an Azathothian strain among the Hyperborean Voormi.
Sfatlicllp is the daughter of Zvilpogghua. She is the wife of a Voormi and their offspring is Knygathin Zhaum.
Shathak is the wife of Tsathoggua and the mother of Zvilpogghua.
Ycnágnnisssz is the being from the dark star Xoth who spawned Zstylzhemghi by fission.
Zstylzhemghi (Matriarch of the Swarm) is the offspring of Ycnagnnisssz, the wife of Ghisguth, and the mother of Tsathoggua.
Zvilpogghua is known to the American Indians as Ossadagowah. He usually takes the form of an armless, winged, bipedal toad with a long, rubbery neck and a face completely covered in tentacles. He currently dwells on Yrautrom, a planet that orbits the star Algol.
In 2005, Chaosium published a Cthulhu Mythos anthology edited by Robert M. Price called The Tsathoggua Cycle, which comprised the original Clark Ashton Smith stories featuring Tsathoggua, along with tales by other authors in which the entity has a starring role. The short story collection includes: