Tathamet

Tathamet was the second being to exist in Creation. The counterpart of Anu, its creation and death led to the birth of the Burning Hells.

Origins
According to various myths, Anu was the first, and for a time, only, being in Creation. Seeking a state of total purity, Anu cast all evil from itself. However, these elements could not exist in a state of separation, and coalesced to form a new being: Tathamet, a seven-headed dragon that was the sum total of all Evil. Though separate entities, Anu and Tathamet were bound within the Pearl that contained them. There they warred against each other in an unending battle for ages uncounted.

The Birth of Hell
Tathamet died in the final battle it had against Anu. Tathamet's blackened, smoldering husk spiraled into the lower darkness of reality. From his putrid flesh arose the realms of the Burning Hells, and the race of demons that arose from them like maggots from a corpse. Additionally his seven heads spawned the Seven Evils. The strongest of his maws gave birth to the three Prime Evils, while the four lesser heads gave birth to the Lesser Evils.

The only witnesses to their eons-long struggle were Ophis and the Great Red, who were extremely, extremely young at the time. Back when they just started to become aware of their surroundings.