In the dark and in the cold, the heroes stand alone.
There's is a time of survival.
Instead of invention, there is only depravity.
Instead of enlightenment, there is only subjugation.
In the deep west, the peoples once known as halflings have had to adapt there natural tendencies towards surviving the the harsh whims of their overlords - cruel outsiders who do not exploit the small people so much for the resources of their territories, so much as for the dark pleasure of seeing a once carefree people turned to depression and sheer survival. The fields of the halfling homelands, once the place of berries and wildflowers, now get used for slop-fields for their opressors livestock.
In the forests the elves have tried to rebel against the same darkness that tramples the will of their smaller cousins in the west, but their kings and queens have given in to bribery. Theirs is a world of failing militias and forgotten magic. The once grand arboreal kingdoms, spoken of often in the myths of all peoples are now choked in cobwebs and swamplands.
The gnomes, once proud tinkerers and musicians, now toil to produce weapons of siege for their cruel masters and all but the occasional dirge is forgotten by even their most accomplished bards. They have no homes of their own, instead squating in whatever hovels their oppressors deem fit in whatever village or city their services are needed.
As far as anyone knows, the dwarves have largely avoided oppression by driving their society deeper underground. The once insular people have let themselves slip into a full blown zenophobic paranoia. To make matters worse, their recent delvings have awoken a new threat that had long slumbered in the deepest recesses of the Underdark - an ancient mindflayer colony stranded aeons ago when their planar magics had failed them.
The only race not greatly affected by the opression that plagues the other races is the half-orc population that was already scattered throughout human society. As an insult to the humans their new masters have placed the once oppressed half-orcs in what positions of power they had allowed the humans to retain. These new half-orc lords are mostly figureheads in terms of the power they wield, but this has not stopped them from cruelly gloating over their once oppressive relatives.
***
At least that's how I would do it.
