The Altar of Quetz'lun is a temple, within Zul'Drak, dedicated to worshipping Quetz'lun, the wind serpent goddess. 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.
This uses the same graphics transformation as being dead, and works much like the Bash'ir Phasing Device in that while you have the buff on, you are in the phased environment.