Kabar, Harbinger of the Corruptor

Originally, posing as the young human noble Darrin, the Party later learned Darrin was actually a dangerous rakshasa named Kabar. Kabar was behind the Cult of Reclamation's plans to usurp control of Frasnir and Boriss. Eventually, Kabar revealed his true goal: release his dark master, the ancient demon named Lord Zoul, from his prison beneath Boriss. With the help of the cult, Kabar found the Compendium Tributis and deciphered the ancient artifact to learn the secret to unlocking Zoul's divine cage. The party finally defeated Kabar at the fort where Luther made his original deal with Zoul. Upon the fiend's death, Zoul made one last deal with Luther and the two became one as the long forgotten demon lord was released.
Months later, Kabar returned to help Lord Zoul's son, Darius, gather Origin Gems to power a magic ritual to open a rift between dimensions. Kabar infiltrated the elves and mind controlled the elven queen as her "advisor" Phorn Sunmantle. The heroes managed to stop him and kill him once again, but not before Darius stole the Catalyst from Lady Sariel.