Tathamet, The Prime Evil, was a seven-headed dragon of all darkness and vileness. It was known as the Prime Evil and was created as a cast off of all the dark attributes within Anu long before Sanctuary came into existence. Seperated from one being, Tathamet and Anu were both trapped within the same place and they battled with each other for an indefinite length of time. Eventually, the evenly matched adversaries destroyed each other, causing the universe to come into being.
After its death, Tathamet gave birth to the seven Great Evils, each being formed from one of Tathamet's heads while its body became the foundation of the Burning Hells.