From the introduction:
The party saw the newly-carved sign when they first entered the mundane village of Riverfork – “No magicks allowed.” Something strange was clearly afoot; arcane power preceded them too the town and was not welcome.
Two brothers, Herman and Franz Finn, have been exiled from the village, after they were caught practicing the dark arts. They have since holed up in a cave upriver, where they stumbled upon an ancient shrine to Tharizdun.
Corrupted by their dark worship of Tharizdun, they have since taken on the identities of Germ and Vras Vrimm, and have recently conscripted a tribe of local bullywugs to build a dam to control Riverfork’s water supply. With the bullywugs’ assistance, they have been engaged in systematically poisoning the village’s water supply and causing shortages and floods to exact revenge on their exile.
This work has pleased Tharizdun, who has sent them a Barlgura to assist them in their efforts of chaos and evil. They are currently recruiting more bullywugs and other forces with the intent of razing the village in the name of their dark god.
This is a short adventure for 5th level characters.