The High Priest of Scabaras is a Scabarite who is the head of the Worship of Scabaras, the desert god of solitude. He lives in the underground hive in the tunnels of Ullek along with the rest of the Scabarites, and is encountered at the very end of the Dealing with Scabaras quest. In the quest, the player attempts to stop the Scabarite attacks on Sophanem by neutralizing the threat of the Scabarites, but when they reach the Antechamber of the High Priest, he is revealed to be insane and under the impression that the only way to gain solitude is to kill all non-believers.
The High Priest does not need to be fought in the quest. Instead, he must be talked back into sanity in order to subdued, and if the player mentions Amascut, the Devourer, one of two things will happen. Either he will become angry and claim that the player is a heretic, or before attacking them, if the player is able to stop the attack, they can talk to him and try to get him to see reason again. Eventually he will see reason and his head will clear of the Devourer's spells, agreeing with the High Priest of Sophanem not to attack their city, though the Scabarites will still defend themselves if threatened. After being defeated, the High Priest of Scabaras gives the player more notes for their notebook.
Interestingly, the High Priest breaks one of the most important rules of Scabarite culture: isolation. He makes contact with the player, by both speaking to and attacking him/her. This, however, is not completely sinful, as the player seeks him out in his isolation, and he was possessed by Amascut.