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!
The player characters, experienced and somewhat well known, hear rumors and travelers' tales about a distant area being overrun by dragons (or other terrifying monstrosities). The locals have sent them a message, begging for heroic aid.
When they investigate, they discover that nothing of the sort is going on. It turns out that a group of thieves wanted them out of the way so that they could rob them (or someone who would normally receive their protection).