The Underground City in the Big Rubble is largely networks of cellars connected to other cellars. For example, the following shows a network of six cellars connected together through a central cellar. Each cellar might have several rooms.
Meanwhile there might be isolated basements or cellars between them, either never connected to the network, or deliberately blocked off.
The nature of these rooms vary wildly. Some have been the residences of human or troll groups for centuries, and have maintained shrines, sleeping quarters, kitchens, storage, etc. Think like the Cappodocian underground cities of Derinkuyu or others.
Others have been abandoned for centuries, with rubble and perhaps remnants of prior civilizations. These might be the current residences of some less civilized inhabitants – trollkin, cave trolls, bandits, ogres, broo, or even undead. Passageways might lead to walls of rubble.
Among the underground locations I have heard rumored in the inns of New Pavis include:
A complex for the initiates of Black Fang, complete with a temple, sleeping areas, and strongboxes of treasure.
The center of the Cacodemon cult, with centuries of treasure, magical items, and guarded by cannibal warriors.It is said this was once a catacomb where the Pavis Survivors buried their dead.
An underground auditorium now used for depraved blood sports.
A former winery cellar turned into an evil shrine.
A cellar used to store the records of the Arrowsmith Dynasty, filled with lost knowledge.
