It's a new territory. Territories are kind of like the "monsters" of the Expedition Conflict. We'll be exploring that concept a bit more in updates.
Each territory has rules that affect adventures and expeditions. There's also a unique stuff table (for Loot Table 2 rolls) and a huge list of "threats", which are folk, monsters, terrain, and weather that threaten the expedition.
For the Cavernous Underworld you'll get all of those things, just like you get for the Putrid Swamp, Misty Mountains, and Scorching Desert (and six others) in the Vagrant's Guide.
At the moment, this stretch goal is for a PDF expansion. If we really knock it out of the park with the funding, we might consider lengthening the book to include the Cavernous Underworld. Keep watching for updates to find out more about Threats and Territories.