A massive Taleen dungeon that Ilea finds after being forcefully transported from the Taleen transportation platform on the Isle of Garath by Arthur Redleaf[1]. Inside Ilea finds massive structures "as big as mountains", Taleen Guardians in the thousands, many Praetorians, flying Taleen machines, and a golden sphere so large and far away that should couldn't determine its size.
It is later hypothesized by Christopher Volaris that Iz is the very core of the Taleen dungeon system[2]. This is confirmed in Chapter 589[3] which includes a potted history of the construction of Iz and the Taleen in general.