I have recently been working on developing an underdark city for Athas. I was going to give what ideas that I had so far and listen to some community suggestions/feedback. Thanks in advance.
The Sunken City is located in the underdark beneath the crescent forest. It is actually three different communities put together.
The upper city that is built into the stalactites on the roof of the large cavern has a strong dwarven presence and is dominated by their ancient architecture. Despite the city’s age, sturdy stone bridges connect the many structures built into and out of the hanging rock. It is controlled nominally by the dwarves provided that they do not cause trouble with the aboleth rulers.
The middle city is more accessible to most and is built on the islands and large fungi to be found in the great cavern. It is an area for trade and a neutral meeting ground for various groups of Athas’ underdark. It is policed by the kuo-toa, who ferry non-water breathers into/out of the middle city.
The lower city is populated only by aboleth and kuo-toa. Others can only allowed to travel here by invitation of the aboleth. Violation of this is punished by death.
Lower city locations:
Palace of the Ancient-This alien palace is home to all the aboleth (all 4 of them) on Athas. It mainly refers to the huge sleeping ancient aboleth that is hidden in the lower part. The aboleth that anyone would actually meet is the one calling itself “the Voice”.
The Cathedral of the Depths-The kuo-toa temple dedicated to water. The aboleth have “convinced” the kuo-toa that the Ancient is a god and grants them their power over water.
Locations in the Middle City:
Dark Water Monastery-Headquarters of a cult of water worshiper surface race members that the aboleth created in an effort to wipe out silt clerics and ultimately the Dust Kraken.
The Sunless Sea Market-A huge number of exotic shops, stalls, bazaars, and emporiums filled with all manner of goods (legal/illegal/magical).
The Aboleth
These 4 aboleth are the only remaining “active” aboleth on Athas. The rest have either been forced into hibernation or perished (despite the fact that an aboleth shouldn’t be able to permanently perish).
The Ancient is a size huge Psion(Seer)20/ Cleric(Water)20. It spends most of its time doing two things: 1) psychically looking through space/time to locate and find ways to destroy the Dust Kraken; 2) using its power to bring more water to the Sunless Sea.
It is responsible for creating the kuo-toa. In the ancient past it ate a halfling shaper and managed to gain a flawed understanding of shaping. Because of this and regular psionic tampering by the aboleth, the kuo-toa tend to deteriorate mentally.