Xhamen-Dor

Xhamen-Dor, the living cancer, was born in the sewers beneath Carcosa, the alien city that is the home of Hastur. Somehow, this Great Old One was transported to Golarion within a comet, crashing into the bottom of a lake during the devastation of Earthfall. Physically, it manifests as a twisting and vaguely serpentine-shaped mass of bone, hair, and fungus.

The Inmost Blot, as Xhamen-Dor is also known, exists to conquer worlds by infecting the living with itself. Its victims, the seeded, enact its will and seek to spread its influence until the entire world is but an extension of Xhamen-Dor. Then, once only the seeded remain, Xhamen-Dor uses the last of the planet's energy to return to Carcosa, which in turn absorbs the world Xhamen-Dor has brought to it, allowing Carcosa to slowly grow.