Thelamos (5E)

From publisher blurb:

A 5th Edition Mini-Dungeon for 4 Level 4 Characters

This dark, stinking complex is located deep in the earth, buried underneath a major city or town and accessed by the sewer system. Once the hideout of a long-forgotten band of thieves that operated in the city long ago, these chambers have since been discovered and taken over by a group known as The Sons of Arratoi. They have undertaken a number of daring attacks and raids in and around the city, achieving a degree of notoriety but none have yet traced them back to their lair.