Ubbo-Sathla first appeared in the story Ubbo-Sathla (Clark Ashton Smith, 1933).
Ubbo-Sathla is a protoplasmic, featureless being said to have spawned all earthly life (possibly including humans), and to whom all such life will return in the end. Its spawn are endlessly created and cast off about it, with Ubbo-Sathla recapturing and devouring many of them with its pseudopods.
Ubbo-Sathla dwells beneath the ground in grey-lit Y'qaa (or possibly Mount Voormithadreth), guarding a set of stone tablets said to bear the knowledge of the Elder Gods themselves. Many wizards have attempted to gain these tablets, but not one has ever succeeded.