One of the main features of the campaign setting is that everything is local. Nothing is really widespread across all the known caverns and tunnels. So something might be limited or not-exist in one place but common as dirt in another.
This goes for special skills, weapons, armor, equipment, types of magic, etc. If you want to learn some exotic power or get some rare material it's all a matter of finding where to go and surviving the trip there. Lots of non-class abilities and powers can be gained if a PC can just find the right place to learn them.
A port city a bit away from the most travelled areas of the known caverns.
The largest city in the entire underworld. A culture of elegance and art. Beautiful metalwork and powerful magic.
A large civilization that dominates the ocean in the inner world of the endless sun.
One of the main sources of food in the underworld. Miles of open caverns with low ceilings that have hundreds of farms overlooked by hundreds of castles.
DeathFrost Lands- A place of cold and death, where nothing lives. To the south-east of the farmlands.
A isolated cavern far away from any of the main areas of the underworld
A small town in the ore-rich area that produces most of the metal the rest of the underworld uses.
An area of independent settlers, wild magic, and small towns. Mostly isolated patches of minor civilization.