Great, mysterious chambers hidden deep in the earth beneath that enigmatic city of Stoneholt!
The city of Stoneholt is generally considered one of the Wonders of the World, but it conceals a second wonder - a labyrinth of massive sewers and storm drains greater then anything man has built.
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).