Death from Below is an OSRIC-compatible adventure for 1st to 3rd level characters.
Though set in the World of Arkara setting, it is generic enough to run in any fantasy city with a large sewer system.
Death stalks the city of Bondaea, with 17 citizens missing or dead. Clues lead to the sewers beneath the city, where evil creatures have moved in, using the sewers to move unseen, hunting the unwary townsfolk above.
In addition to its adventure content, this module also contains two new monsters and two new magic items.