The Sunless Depths

From publisher blurb:

Beneath the surface of the world, miles down into the crust, lies a network of dark caverns and twisting passages. These massive chambers and narrow crawlways form a vast underground habitat. Deadly monstrosities and extraordinary beasts make this darkness their home, evolving and adapting to survive within the pitch. Deep down where daylight has never penetrated, great wars wage and monstrous kingdoms flourish. Undead cities, and ancient ruins lie amongst mushroom forests and underground lakes. Yet, amidst the harsh surroundings, great spoils await the worthy adventurer.

The Sunless Depths are divided into nine geographically diverse regions, each with its own characteristics, inhabitants, and history.