Ok, since people seem to want for a setting made from scratch, instead of an "What if" situation with an already stablished, here are the ideas I believe would work best:
Decaying Society
-Taint has always existed, and people societies ways to fight it. But since cults kept appearing trying to harness the power of Taint to their cause, leaders decided that they could burrow taint under secrecy and ignorance. Anti-taint measures were integrated into the popular traditions and law, but the true nature of taint was obscured, deemed taboo, something that it was best not to be spoken of. If Taint appeared somewhere, it was to be dealt with in a discrete way. But still, something doesn't feel right. Tainted items keep being found underground. Pockets of tainted terrain are being found in the darker places of the world, slowly spreading outward. Seemingly normal people are suddenly overloaded by Taint and wreck havoc in the form of fiends. Several times the dead raise back on their own. Clearly the system isn't working anymore. Somebody is actively working to spreading Taint behind the scenes. Can you discover who?
Post-Apocalyptic
-Nobody knows how, or even exactly when, but it happenned. Most of the world is covered in heavy Taint, kingdoms and empires collapsed, fiends and undeads are at the top of whatever passes for society in this forsaken world, altough for the most part is a dog-eat-dog world. Only a few "safe" areas of precious untainted land remain, hiding some thousands of "pure "people. But hiding is not enough. For now the forces of Taint are still disorganized, fighting against each other for power and resources. It's only a matter of time until one of them proves stronger than the others and unites them to finish covering this world in Taint. You must strike out before that. You must go out there and start purifying the land, cleanse the fiends and unliving!
Combination of the above
-After the Taint apocalypse, all seemed lost. The fiends and undeads are too many, too strong, and whatever safe areas remain are being slowly infested. One last hope remains: a time portal is created/found to return to the past, to try to prevent this hellish future. Collect clues on the "present", then go back to the past and prevent the Taint sources from spreading before they wrecked too much havoc. The time portal power is limited, you can only go back to a fixed time in space and time and in reverse, it can only transport a few special people, but it's your only hope.
