Terror from the Deep

From the introduction:

This adventure can be can be easily customized to a party of any size or level, or run as a level 6 adventure for 5 players. It can work as a small side adventure in an existing campaign, as a standalone one-shot, or the start of a new adventure. Use the links to the tools in their relevant section to customize the adventure to your party.

Fairlun sits in the shadow of a large ruined castle, and is known for its mines and industry. The city is ruled by a half-orc priest of the local religion that keeps the rest of the populace under strict control.