This dungeon is a bastion of the Order of the Raven Hand, a human cult that worships shadows. They are trying to lure unwitting humans to be sacrifices for the shadows.
There are several items in this dungeon that you need for the The Caliginous Bethel.
The first part of the dungeon is full of human Cultists and some Elementals (of all 4 elemental types).
There is a fairly complex series of levers you must work to access the bottom area (see the map and The Caliginous Bethel for more details).
The bottom area after you jump in the Sacrificial Pit is populated by Shadows.