Described as a being of darkness, whether this refers to Rhogog's nature or its physical appearance is unclear. In any case, Rhogog is usually encountered when manifested as some form of colossal, leafless tree with a single great eye embedded in the trunk. It is said to be so hot to the touch that physical contact will actually burn flesh.
Long ago, possibly somewhere in the British Isles, Great Cthulhu clashed with his half-brother Hastur. During the conflict, some of Cthulhu's blood was spilt; this blood was subsequently harvested by a group of Star-Spawn for reasons known only to themselves. The Star-Spawn then proceeded to implant an 'entity' into the blood to protect it; the resultant creature spent long ages in dormancy before finally emerging from its sleep as the Great Old One Rhogog.
One tale tells of a time when Rhogog met a human named Patrick, with whom it debated at length the differences between good and evil. It finally decided that the concepts of good and evil were irrelevant, and proceeded to consume the unfortunate human.
- Sacristans of Rhogog (1991), by Michael Saint-Paul.