Ahriman is a demon who at one point had a large amount of influence in hell. And even waged a war to try to take it over. Unfortunately he was betrayed at the last moment by his generals, and killed by Asmodeus. This was not the end for him though. In his death he became a Vestige, a being beyond the touch of death. Several thousand years later(now current time), he was brought back through a ritual done by Adria using Lagonis.