The Slumbering Caves
Once you enter this zone you can not use the Rod of Recall until the Guardian is defeated!
Entered from the Palace of Fumes. Four levels of descending rock caves, in the shape of large concentric circles.
The bottom level is Amakthel's Prison which has the final campaign boss, the Sher'Tul High Priest. The boss will fight you while the body parts of Amakthel will spawn tentacles of various types to make life hard for you.
You can't kill any of Amakthel's parts, and they have near-infinite life, so don't bother.
Once you've defeated the Sher'Tul High Priest, you can use the Rod of Recall, but only on the final floor.