The sacrificial pits of Azjol-Nerub were used long ago to sacrifice enemy forces in exchange for success in battle. They are now used by acolytes of the Scourge who wish to serve as the eyes and ears of Ner'zhul, allowing them to shed their bodies and take on the forms of invisible shades. Shades are detectable only by magic, making them excellent spies.
Pit of Fiends
This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials. It should not be taken as representing official lore.
Being a nerubian pit full of bones, the Pit of Fiends might be a sacrificial pit, though it has nothing to do with shades.