A rare and diseased jewel, a canker fallen from the heavens
Fist sized diamond made famous by the thief Tolky. Stole the ‘rock’ from the Archmage Olikan and made it famous when his hidden thief’s den was found and destroyed by Olikan. The rock was never found though and Olikan searches for it to this day.
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).