A thousand years ago, the world's population was on the eve of reaching 10 billion. The dragons were nearly wiped out, the monsters and evil races of the realms were pushed to the frontier, and enchanted crops gave bountiful harvests year after year, providing for the goodly races an unprecedented era of prosperity. The explosive growth of humans, elves, dwarves, and halflings began to plateau as they became less fertile.
Seven hundred years ago, the famines had begun in earnest. The magic that had sustained mankind began to fade from the world- the infrastructure of advanced civilizations began to fail and the farmland withered unto dust.
Three hundred years ago, clerics could no longer hear their Gods at all. They still drew scant power from the divine, but both divine and arcane magic was once again becoming a thing of myth- something unknown, something to be feared.
A hundred years ago, the barriers separating the planes began to weaken... Demons and devils began to roam the earth. There are places where time and space warps, where the planes break like waves upon the sand.
Now, less than 300 million people are estimated to eke out a living in the wastes. Many are infertile, others struggle to forage food from the toxic forests, all the while weathering the constant threat of the hellish invaders.
-High point buy (NPCs will probably have class levels and will be as tough as their world)
-Tough enemies will sometimes make outright fighting a pyrrhic solution. Think tabletop Dark Souls.
-Possibly... Tieflings as a playable race, though perhaps nerfed to reduce LA. They are one of the few races reliably fertile, and have been seeded into the material plane by the invading fiends to 'muddle the gene pool', most are unfortunate products of rape and inherently disdained.