Nergal (archdevil)

Nergal is an outcast archdevil who resides in Avernus. In an attempt to learn the secrets of silver fire, he captured Elminster while the wizard was in Avernus and exhausted from sealing a planar rift between Avernus and Shadowdale.

Nergal subjected Elminster to horrific suffering, all the while stealing his memories using a mental link, until Nergal's body was destroyed by the Simbul (his life-force seemingly escaped).