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 the past, 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 is a debate between Rhogog and a sacrificial victim named Patrick about good and evil. The tale ends with Rhogog deciding that good and evil not being a matter and simply killing Patrick, but it does not explain whether or not the stars are made right.