From the back of the book:
Dungeon World is a d20 System campaign setting that turns the idea of fantasy role-playing on end. The second follow-on to he wildly popular Dungeon World Universe campaign sourcebook, this book takes Players and their characters deep into the Catacombs, where even the fiercest and strongest denizens of Dungeon World fear to tread. Make no mistake. This is the place even gods fear to look.
Behind, Beneath and Between...
...the levels of the Nex and Kerse, the Enemy Capital, lie the Catacombs. Listed on no map and rarely visited by even the Caretakers' servants, the Catacombs are home to the degenerate, the dejected and the derided. Rarely does one find the Catacombs - they find you!
A Hell Among Hells!
This is not a place for the meek of heart or those who fear all that is dark and evil. Heroes have come, but have rarely left. Each of the Catacombs is worse than the last. They can be explored by those brave enough, or foolish enough, to try, but never mastered. They are the depository for the hopeless and the helpless - those too pitiful for even the Nex - but especially for the heartless.
The Place Even Gods Fear...
...for one of their own is imprisoned here. It is the place that the gods send their unwanted, and even those from among their own number who can no longer be trusted. Trapped here within the Catacombs with no hope of ever escaping, powerful gods and demon lords have made realms anew for themselves. But how long can they remain cooped up before the pressure builds up too high?