Mischievous Monsters

From publisher blurb:

Several leagues off the main road is a portal to the realm of fairy; a ring of standing stones resting in a clearing surrounded by ancient trees. Once a year, from the full moon to the next full moon, this portal opens and allows the fairies to cross over, to wreck havoc and cause mischief in the mortal realm.

This adventure pits the characters against a group of mischievous fairies, who raid the inn the characters (henceforth, the Party) are staying at and steal the goods of a wealthy trader. The trader hires the Party to track the thieves down and recover his goods, and awards them handsomely to do so.