Real world scientists have accidentally opened a tear in the fabric of reality, transporting their laboratory into the game world. Unable to repair and/or refuel their equipment in this (to their minds) primitive world they are now trapped.
The local band of 'nasties' (goblins / orcs / whatever) lives in relative peace with the local population.
Along come the PCs and go through their usual heroic monster bashing routine, wiping out the nasties and pinching all their stuff, then continue on their way.
Problem is, they don't kill ALL he nasties. The survivors want revenge and, after spending a few months recovering, start to take it in their own inimitable style (which is not unlike that of the PCs come to think of it).
Next time the PCs are in the area they find themselves VERY unpopular with the townsfolk.
A plague has hit the local area.
In humans it affects only the most vulnerable, the children and the very old, and even than it's little more than a summer cold. 24 hours of sniffles and then it's gone, barely noticeable really.
To sheep however it is fatal and the whole economy of the area is in serious trouble. If this keeps up the area could well be facing famine.
Somebody (enter the PCs) must find the rare herbs needed to make a cure.
The nations of the Kolm surpasses all other barbarians in their wilderness of life. Thoug they do just bear the likeness of men, of a very ugly pattern, they are so little advanced in civilization that they make no use of fire, nor any kind of relish, in the preparation of their food, but feed upon the roots which they find in the fields, and the half-raw flesh of any sort of animal. I say half-raw, because they give it a kind of cooking by placing it between their own thighs and the back of their horses. They fight in no regular order of battle but by being extremely swift and sudden in their movements, they disperse, and hen rapidly come toghether in loose array. They spread havoc over the vast plains and flying over the ramparts, they pillage the camp of their enemy almost before he has become aware of their approach. They are the most terrible warriors for when in close combat with swords and flails they fight without regard to their own safety, and while their enemy is intent upon parrying the thrust of the swords, they will entangle him with their chains so that he loses all power of walking or riding.
Excerpt from "The peoples of the world" By Taklamarian court-scholar Guliman Amon.