Nergal was a changeling wizard who impersonated a bishop of the Sovereign Host, Bishop Avery, in order to convince Baba Moonweave to help them access an account that's contents were allegedly left to the Church of the Sovereign Host. Nergal battled Baba and Gaius Tanaquil at the Hollow House Inn in Sylbaran, and was defeated. She tried to flee but tripped and fell out of a window to her death. Gaius made sure she didn't survive the fall with an arrow to her chest.
Nergal mentioned something about "Vol" and a "dark commander". Perhaps she is in league with the cult known as the Blood of Vol. Her companions wore tunics that had an emblem for the Order of the Emerald Claw. She may have been involved with the Order as well.