Civilization has literally grown up around a world-spanning megadungeon. The dungeon has influenced society in deep ways. The races on the planet may have even originated in the dungeon, thousands of years ago.
1. The dungeon is older than society.
2. The dungeon is enormous, potentially spanning the entire world, penetrating the surface in scattered places (not everywhere).
3. The dungeon has a seeming alien intelligence, grows on its own, and reaches out like a tentacled beast, until it finds the surface with one of its arms.
4. The dungeon disgorges creatures and magically warps things it touches.
5. The dungeon probably disgorged most of the D&D races at some point in the past, and they adapted to life on the surface.
6. Wars were fought between the surface and the underground, eventually beating back the swarms of evil the dungeon was vomiting onto the surface. That was hundreds or thousands of years ago, and things are waking up again.
What is this society like? How is it different than a typical D&D society?