The Ziggurat is an ancient structure below Whitestone, formerly a temple to Ioun. After the Briarwoods seized Whitestone, they took over the Ziggurat and prepared it for a ritual to Vecna. It is somewhat implied that the Ziggurat was what attracted the Briarwoods to Whitestone in the first place, along with the magical residuum from the whitestone mines.
Although the Briarwoods' ritual did not completely succeed, it did leave behind a strange, hovering black orb that sucks in anything that touches it and disables all magic within a certain radius. Because this dispelling effect provided a measure of protection against magical espionage, the Ziggurat was subsequently used as a war council room by Vox Machina and their allies in Whitestone.