Sethaz was the stepson of Theodore Kaczynski, who was imprisoned in and had fled California in the confusion following the Change. Kaczynski became the head of the Church Universal and Triumphant and married the previous leader, Sethaz' mother. Martin Thurston thought of him as a "middling man, middling height and coloring and features."
After Theodore Kaczynski became insane, Sethaz interpreted his pronouncements for the CUT faithful. When Kacsynski died, Sethaz ascended to the position of Prophet.
He declared himself to be the "Scourge of God."
Sethaz and Martin Thurston discussed the progress of the war against the Allies. Martin was disappointed in their progress; Sethaz was more confident. When Martin mentioned the Sword, Sethaz screamed; Martin realized that he had been trapped by his "co-commander" and that Sethaz was demonically possessed
Sethaz was present at the Battle of Horse Heaven Hills. During the engagement between the Sword of the Prophet and the Portland Protective Association, Artos saw Sethaz. Time seemed to stop for Rudi and when it restarted, Sethaz was gone. When Artos drove the Sword into the ground at Lost Lake, he had a vision of Sethaz screaming, beating himself on the temples, and bleeding from the eyes.