The surface is lost. A magical and technological apocalypse rendered the surface completely uninhabitable thousands of years ago, and the final vanguards of all the races descended into the darkness long ago in order to survive. All thoughts of returning to the surface or rehabilitating it to sustain life again have long since been quelled; the darkness is home now, the twisting passages, vast caverns and unimagined wonders of the underworld of Behldazan.
The elves were the first to descend, followed by the gnomes, then halflings, then finally humans. The half-breeds and other races made the trek as well, though not without going through profound changes. The splintered intra-species variants have mostly made up and banded together, learning that division and isolation mean certain death in the Underworld. The dwarves, natives of the caves, begrudgingly ceded many of their lands to the newcomers and have become a shadow of their former glory.
Even after thousands of years of adaptation to life in the caves, all is not as well as it seems. Holdouts of racial supremacists cause trouble in human lands; ancient feuds between elves and dwarves still simmer; the brutal native inhabitants of the caves still test the defenses of the races from time to time; and lurking menaces brood in deep undiscovered caverns, biding their time before making themselves known to the hapless newcomers.
The Great Exodus is long over, but the caves will never be truly tamed, and memories are long in the dark. What mark will you leave on the history of Behldazan, and do you have the mettle and drive to survive the mysteries of the Underworld?