Kassogtha is a Great Old One of the expanded Mythos, created by Joseph S. Pulver Sr. for his 1999 novel Nightmare’s Disciple. She is told as Cthulhu’s sister and mate, and the mother of the twins Nctosa and Nctolhu. Her form is a ceaselessly writhing mass of tentacles, and she is also named the Coiled One and the Leviathan of Disease.
What defines Kassogtha is sickness itself. Like a parasitic disease she seeps into living things to corrupt and mutate them, and at times she merges her body with another entity, that union giving rise to new offspring.
The provenance is worth stating plainly. Kassogtha is not a figure Lovecraft wrote. She was made by another author more than half a century after his death, and this — later creations circulating alongside the original texts as though they belonged to one catalogue — is a condition peculiar to the Cthulhu Mythos. The way the shared world grows is itself a kind of proliferation.
Her place in the family tree is a later addition too. The relation to Cthulhu as sister and mate, the twin offspring Nctosa and Nctolhu, the sisterhood with Cthylla: none of it has a basis in the original stories, and each hardened by cross-reference inside the expanded Mythos. Reading Kassogtha is therefore less a matter of reading a deity than of reading how a catalogue enlarges itself.
Creation and destruction as one act — it shares a register with our world’s motif of proliferation, the debt that swells as it spreads.