From Exalted - Unofficial Wiki
The term "darkbrood" refers to numerous races of hideous beings that live under Creation (not the Underworld, but actually deep underground in creation). They are being held at bay by the MountainFolk, who wage an endless war to prevent them from reaching the surface. In 1st Edition, they were detailed in Exalted: the Fair Folk. It is likely that they will be updated to Second Edition in the Scroll of Fallen Races.