If you want to know who spawned all the demons in the Diablo universe, it's this guy right here. Tathamet was created when Anu cast off his impurities, which manifested into this seven headed dragon. He is known to be pure evil and vile, so much in fact that when Tathamet and Anu finally destroyed each other, each of his heads become one of the Great Evils.
His three prominent heads become the Prime Evil while his other heads became the Lesser Evil. The Burning Hells is also known to have spawned from his body.