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.
While traversing the tractless deserts of Shezmu, the pc's come across a travelling caravan led by Vual, the Demon-Camel, the Lord of Lust, the Thrice-Humped...Vual is cursed to only be able to assume one form outside the Abyss, that of a rather large, golden-skinned three-humped camel,with a frog-like, black, barbed tongue......
Vual is the demonic manifestation of Lust...his followers, a cult of truly disturbed individuals, engage in bizarre orgies, travelling the desert towns, "entertaining" the desert folk with their perverse antics...
Vual's Rapture is what the cultists call their monthly festival, where they perform perverse rites to honor Vual...ofcourse folk come from great distances to witness the festivals, despite themselves, and the Cult of Vual welcomes all...
Surprisingly for such a hideous creature, Vual has the power to beguile and charm with his magical voice. His voice will always sound to pcs as the voice of the person they most desire...his followers also gain this ability slowly over time...
When the pc's meet with Vual, he tries to seduce them into joining his cult. If for whatever reason the pc's resist..well then... Vual would probably kill them for sport, as a lesson for those who "resist Vual's love"....