Three years into their exile, Ganth once again went mad and lead a charge on the Darkling itself. There, Ishtier saw the Darkling and decided, like Gerridon, to give his loyalty to it.
Ishtier betrayed his house and lord by abandoning them and running away south to Tai-tastigon, with Ganth's old mistress, the Kendar woman who was carrying Ganth's son Bane. In abandoning them, Ishtier left his younger brother Anar, a scrollsman, the task of filling his role as a priest and protecting the keep from the nearby force of Perimal Darkling.
In Tai-tastigon, Ishtier takes position as High Priest at the temple of the Three-Faced God. After Bane was born, Ishtier let the boy believe he was his father.
After seeing all gods in Tai-tastigon, Ishtier set out to experiment. When Abbotir told Bane he was to become a thief, Bane turned to Ishtier, who removed Bane's soul so as to allow him to act without honor without dishonoring his soul. Rather than protect that soul, Ishtier uses it to create the Lower Town Monster. Isthier believed he had created another Tastigon god, though in fact he had not, as a soul-based monsters is something rather different.
At the end of her time in to Tai-tastigon, Jame confronted Ishtier. The God-voice spoke through him, and left him mad, gnawing on his own fingers.
A mission of Kencyr to Tai-tastigon retrieved him and brought him back to the Priests' College in the Riverland. He was sick most of that winter and spring, but did eventually recover.
Jame met him again in Seeker's Mask, in the Priests' College in the process of rescuing Kindrie. She demanded a judgement from the Three-Faced God and the God-voice spoke through Ishtier again, sending the man truly insane.
↑God Stalk, Book I: Tatters of Dusk, "Chapter 3: Into the Labyrinth" — "Ishtier, Highborn priest of the Kencyrath, stood in the shadow of his god, watching her with hooded eyes."
↑God Stalk, Book III: Shroud of Days, "Chapter 13: Three Pyres" — " 'All Shanir must possess at least a touch of the Highborn strain, but soul-carriers like you and Ishtier need blood as pure as it comes.' "
↑God Stalk, Book III: Shroud of Days, "Chapter 14: Untempling of the Gods" — " 'I went into exile with the Gray Lord. I saw the face of darkness and know that in all the Chain of Creation there is nothing to equal it.' "
↑Blood and Ivory: A Tapestry, "Among the Dead" — "The garrison had been some three years into its exile then […] again followed their lord headlong into hopeless battle, this time against primordial darkness itself. […] the slow churn of mist under that cliff of shadow that had confounded and dragged them apart. […] It was also when they discovered that their priest, Ishtier, had run off, taking his lord's Kendar mistress and his own priestly powers with him,"
↑God Stalk, Book I: Tatters of Dusk, "Chapter 3: Into the Labyrinth" — "Before Jame's time, this had been the responsibility of the scrollsman's older brother, a priest of great power and knowledge; but one night this man had fled with a female companion, leaving his inexperienced kinsman to assume the terrible burden alone."
↑God Stalk, Book III: Shroud of Days, "Chapter 14: Untempling of the Gods" — " 'He trusted you because you brought his mother, once the Gray Lord's mistress, down out of the Haunted Lands, because he thought—and you let him think—that you were his father.' "
↑ 7.07.1God Stalk, Book III: Shroud of Days, "Chapter 14: Untempling of the Gods"
↑Seeker's Mask, "Part III: The Riverland", VI — " 'L-lord Ishtier has been at the Priests' College all winter, ever since the mission returned from Tai-tastigon with him. H-he was ill until recently.' "