From the back of the book:
At the World's Core
Another world is hidden below—a world that slumbers under the foundations of mortal cities, dreams below the deepest roots of the oldest forests, and plots in hidden places as far below as the mountains are high. These endless caves have many names. Cold Hell. Evernight. The Hunting Grounds. Yet to those who dwell within they are known as the Darklands.
Into the Darklands explores this mysterious and deadly realm of caverns and secrets, from the numerous hazards that plague the deeps to the strange and sinister races that dwell therein. Within these pages you will find maps of the major entrances to the Darklands throughout Golarion’s Inner Sea region, as well as dozens of locations hidden within three distinct realms of the deep. Tables to determine random dangers and wandering menaces, new languages and exotic hazards, and all manner of monsters, including the degenerate morlocks, the wormlike seugathis, the destructive vemeraks, the blooddrinking urdefhans, and the sinister serpentfolk await discovery within!
Just remember—in the Darklands, the night lasts forever, and the denizens of the depths never sleep.