Each time mankind ventures into a new realm of exploration, he returns with tales of incredible creatures. Be it from the new continent, the tops of the mountains, or the depths of the seas. It is the acme of foolishness to assume that space would be empty of lifeforms. The forms that these new biologies took simply defied conventional physics or chemistry
Methinks we have hugely mistaken this matter of Life and Death. Methinks that what they call my shadow here on earth is my true substance. Methinks that in looking at things spiritual, we are too much like oysters observing the sun through the water, and thinking that thick water the thinnest of air. Me thinks my body is but the lees of my better being. In fact take my body who will, take it I say, it is not me.”
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.