After the death of the cult's leader Amon Tillado, Sergei took over the cult and became the High Priest of Avoozl. He led them to cave that would become the Cave of the Dark One within Mount Malign and there they began to use the Dark One Rituals to summon Avoozl into Gloriana.
When Erana and Piotyr arrived to stop the summoning, a great battle ensued. During the course of the battle, the cultists were twisted into terrible Chernovy -- though none more twisted than Sergei himself. After Erana had banished Avoozl -- and herself -- into the Place Between Worlds, Sergei fled deeper into the caverns with the Essence Ritual and disappeared. As the years went by, his mutations became even more horrific and soon the Last Borgov was the monstrosity called the Pit Horror by local legend.
When the Hero arrived in the cave, he narrowly escaped becoming the victim of the Pit Horror. When he returned to the cave at the bidding of the Dark Master and Ad Avis, he stole the Essence Ritual from the Pit Horror (and, if the Hero was a fighter or a paladin, slew the Pit Horror in the process).
Sergei Borgov, the Pit Horror, appears in Quest for Glory IV: Shadows of Darkness.