Rat King's Sewer (OSR)

From the publishers blurb:

Rat King’s Sewer is a dungeon crawl with a mystery set in the sewers beneath the city of Eastgate in the Borderlands Provinces. Found on the eastern side of the continent of Akados in the world of Frog God Games’ Lost Lands, Eastgate is a bustling city at the mouth of the Amrin River. As most of the adventure takes place in the sewers, and nearly all of it in an urban setting, wilderness orientated characters might find themselves at a disadvantage. Then again, the sewers are a maze and filled with all manner of deadly creature, the ability to talk to cockroaches might just prove useful.

Rat King’s Sewer is suitable for characters level 4-6.