Beneath the White House are Aggah-Shan’s catacombs. This ancient lair actually predates the construction of the gambling house above by at least several decades, possibly more. It can also be reached from the Mrathrach Machine below.
Design Note: There is a map of the Lower Level of the White House on p. 311 of the Ptolus sourcebook. The maps here are designed to supersede that map.
