Manoroth is a Balor Demon of great power. Idella tricked a low level cleric of Nerull and perhaps an Archmage as well into releasing Manoroth, the balor from what the party has learned killed the cleric and his followers but if the Archmage was involved it is unknown, his body was not recovered nor has the party been able to find him to talk with him. From what the party does know Manoroth is after an artifact of great power.