Rhogog (The Bearer of the Cup of the Blood of the Ancients) is a fictional deity in the Cthulhu Mythos. He is the creation of Micheal Saint-Paul and first appeared in his short story "Sacristans of Rhogog" (1991).
And as the child searched among the wicker-bushes, he came upon a great tree, blacker than the deepest void in all creation. The child, trembling with primal fear, touched the bark of the tree, and found that the wood felt as if it were aflame. Terrified, he struggled to pull his hand away, but found that the branches of the tree were holding his arm fast. The child shrieked in fear and in pain as a branch began to split his stomach in two, and his childlike voice was replaced by the howls of a being of unimaginable hatred. The woods trembled. Rhogog was still not satisfied.
—Sacristans of Rhogog [1]
Rhogog is a Great Old One and was born when Cthulhu's blood was spilt upon the ground during a clash between Cthulhu and his half-brother Hastur, hinted to have occurred around Great Britain or Ireland. Intending to use the blood for some unnamed purpose, several Star-Spawn put an entity into his spilt blood, an entity which would ensure that the blood would never be harmed. This entity lay dormant, gathering power, and eventually became the being called Rhogog.
Rhogog is a being of darkness, although normally is seen in the form of a great, black tree. No matter what shape he takes, he is always very hot to the touch. It is unknown if this is simply due to Cthulhu's blood within him or some other aspect of his dark nature.
In Michael Saint-Paul's short story "Sacristans of Rhogog", three Cthulhu-worshipers devise a plan to harness Cthulhu's blood from Rhogog in order to place the stars in correct order. However, the main conflict in the story is a debate between Rhogog and a sacrificial victim named Patrick about good and evil. The story ends with Rhogog deciding that good and evil don't matter and simply killing Patrick, but it does not explain whether or not the stars are made right.