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.

Overview of the Underworld (how it all is laid out and fits together)

Wild Lands
An area of independent settlers, wild magic, and small towns. Mostly isolated patches of minor civilization.

A port city in the middle of nowhere, about halfway to everywhere else. Was built around a lich tomb for the express purpose of looting it. That goal has continued through the centuries. It’s a really well protected tomb.

Fort Lawful (working name)
A once abandoned fort that is being rebuilt by a band of adventurers.

A small village in a remote area of the wildlands where master blacksmiths work on the remains of weapons and armor from abandoned battlefields.

Golden Fields
One of the main sources of food in the underworld. Miles of open caverns with low ceilings that have hundreds of farms overlooked by countless castles.

Land of Frost-Death
Caverns and tunnels filled with nothing. Only cold and death where nothing lives. To the south-east of the farmlands.

Jade Lotus City-State
The largest city in the entire underworld. A culture of elegance and art. Beautiful metalwork and powerful magic.

A isolated cavern far away from any of the main areas of the underworld. Filled with mutated creatures and plants of a variety and density not seen anywhere else in the underworld.

Ivory Dominion
A large civilization that dominates the ocean in the innerworld of the endless sun.

Saurian Island
A large island (perhaps mini-continent) inhabited by intelligent dinosaur people. One of the few places on the great ocean that isn’t under the sway of the Ivory Dominion.