Chahalheel

Chahalheel is a giant, mutant organism living below Seattle.

Background
It is never made explicitly clear exactly what Chalhalheel is. The visions experienced by Timothy Ashford when he communed with Chalhalheel imply Chalhalheel is some kind of alien lifeform that came to Earth long ago. It lacked sentience and lived peacefully below the city until the Great War, where exposure to FEV and constant expose to radiation mutated it. Chahalheel became capable of "bonding" with humans. On it's own, it was harmless, but if it bonded with a person it granted them superhuman powers and an unnaturally long lifespan. Both Kheiro and the High Lama partially bonded with Chahalheel.

Kherio plan was to destroy Seattle, exposing Chalhalheel to the air. From there, he could bond with it and then rule the world as a god. There is no guarantee his plan would have been successful.

Some how the Issaquah were aware of Chalhalheel's existence, and worshipped it as a "Dark God." It's existence inspired them to create the precursor to Kheiro's Prophecy of Four.