Wraith Overlord: Terror Beneath the City State (1981)

From the Introduction:

"Any good Judge has probably already realized that a city the size of the City-State of the Invincible Overlord must ahve sewers and storm drains and that these would make a fantastic place for adventures. This product describes in depth what is there: no only the sewer system, but basements, secret tunnels, and even the city Jail. It describes the whole situation, people, places, and things as they stand now. Who knows how things will change once your group has become involved?

The sewers are not the only available avenues of travel below the city; there is also an entire maze of tunnels dug by the Goblins and others so old that no one knows where they come from. The creatures and people who inhabit this interconnecting maze of sewers and tunnels rarely see the light of day and live in a perilous world were only the toughest survives."

From the front cover:

"Subterranean City-State adventure! Explore the terror-filled tunnels! Fabulous treasures and unique dangers await the intrepid adventurer who dares to brave the depths beneath the city!"