Zagraal is only known as a Demon, responsible for fueling the Khazra (or Goatman) with demonic power. The Khazra served the demon lord in exchange for being empowered by his chaotic magic. The Umbaru sent their Witch Doctor heroes to combat the demon, and in the end Zagraal was slain.
Zagraal's ranking amongst the other Demons is unknown. As he was powerful enough to empower and turn the five umbaru clans into brutal Goatman demons, Zagraal could very well be one of the Lesser Evils, although he is never mentioned. It would also be unclear what his 'domain' would be (like Pain for Duriel, Anguish for Andariel). Still, it's safe to assume Zagraal was a powerful demon. How he managed to manifest himself in the mortal world remains, like any other information on Zagraal, a mystery.
| Prime Evil — Tathamet|
Prime Evils — Diablo • Mephisto • Baal