Central chamber somewhere deep in the magicians' tunnels of Nasheen where magician hopefuls are tested and given the title of full magician. It marks a neutral point between areas administered by magicians and that governed by the Plague Sisters.
From God's War:
Rhys entered the plague hall. Yah Tayyib and two other magicians sat at a large circular stone table at the center of the room. Three Plague Sisters, the hems of their white robes dripping with spiders, sat across from them. Like Yah Tayyib’s operating theater, the plague hall was a cavernous room lined with jars of mostly human organs. And like the magicians’ quarters, the whole room hummed with the sound and feel of bugs. Rhys’s skin prickled. He had waited some time for this.