In the Age of Sorrows, the Underworld is a dark reflection of the living world, with living (or once living) kingdoms represented in every nook and cranny of the landscape. The great exception to this is the city of Stygia, located in the heart of the Isle of Sorrows, the Underworld's reflection of the Blessed Isle.
Stygia is a grand necropolis with no counterpart in Creation; it is the center of the cult of the Dual Monarchy, the location of the Calendar of Setesh and the entry to the Well of the Void. As such, it is the most important place in the Underworld qua the Underworld; the ghosts who live in Stygia live there for its power, its respect, and because the affairs of the living are less germane to them now.
Given Stygia's importance, it is a key city. Since the Great Contagion, Stgyia has been used a council-chamber for the Deathlord, who conquered the city and now rule it from the shadows, using the Dual Monarchs as their figureheads.