Zirithian was a drow fighter from Phaervorul. Over time he turned away from the worship of Lolth, instead embracing the demon lord Orcus. Finding a way into the Abyssal domain of Deadhold, he claimed the fortress of Hordethrone for himself. He acquired the sword Nightbringer, and was transformed by Orcus into a powerful vampire. The demon lord promised to raise Zirithian to the station of exarch in exchange for a large quantity of souls.
Working with other agents of Orcus within Phaervorul, Zirithian sent an army of undead to ravage the city to provide souls to Orcus. However, the Challengers of the Unspeakable became embroiled in his plans when they arrived in the city looking for Theriana. Planning to present Zirithian's head to Matron Urlvrain, they entered Deadhold and confronted Zirithian within his ziggurat in Hordethrone. There, after a long and heated battle, Zirithian died when Sarahlia sank an arrow into his chest.