From the introduction:
Diguo-dashu, a subterranean civilization of warring ratfolk, sprawls throughout the Sekamina layer of the Darklands beneath central Tian Xia. In the year 6751 ic (4251 ar), hordes of ratfolk invaded the surface empire of Lung Wa. Unable to seal the entrances to the Darklands faster than ratfolk demolitionists could create new tunnels, and unused to fighting a foe that could swarm on to the surface from almost anywhere, Lung Wa struggled to contain and repel the invasion. The ratfolk seized control of a dozen cities before the Imperial Army managed to halt the invasion and eventually drive the invaders back underground, sealing at least 20 Darklands entrances with powerful earth magic. The legendary sorceress, Hao Jin, the Ruby Phoenix of Goka, orchestrated one of the more dramatic sealings.