Altar of Quetz'lun
Right before her physical body was destroyed, Quetz'lun created her own private underworld nightmare in which she dragged all her former worshippers and high priests into. Quetz'lun was unable to pull her prophet in however.
Beginning with the quest , upon entering this region, the character will gain the Ghostly buff, "leaving their mortal form behind" and going into Quetz'lun's private underworld.
Altar of Quetz'lun Digsite
The Altar of Quetz'lun subregion (which includes the altar itself and the courtyard in front of it) is a troll archaeology dig site. It should be noted that the elite guards are initially hostile, and become neutral through completing quests relevant to the area. The guards being neutral greatly simplifies surveying in the area.