The party is walking through the forest at night when they come upon a clearing. Half a dozen black-robed corpses lie scattered across the ground, and a pentagram of blood is shoddily drawn in the dirt.
It seems these people summoned something they could not control. Whether or not the demon returns is up to the GM, but it would be just as rich if the demon never returned, and for the rest of the night, every stray sound or odd shadow will be jumped at!
A party of adventurers walk along late on an open plain, on a moonless night. Abruptly, War screams, the clanging of metal and death-cries are heard. It is an open plain, and nothing is seen, but the sound of a huge battle is all about them. The sounds continue for a half hour before stopping as suddenly as they started. What was it? Perhaps the ghouls of a long-gone battle, reliving their unfortunate last memories...
In some remote regions of the lands, voices of spirits and ghosts can be heard whispering in the wind, and wispy movements can be seen out of the corner of the eye.
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.