Slums Sewers Overview - Baldur's Gate 2
The Slums Sewers is a small dungeon in the Slums portion of Athkatla that connects the Copper Coronet to the Slaver Stockade. Usually you'll enter these sewers during the Free Hendak and the Slaves Side Quest which you start inside of the Copper Coronet. Going through these sewers is the safest route into the Slaver Stockade.
For the most part this is a transient map that you'll quickly be traveling through, the only notable thing here is the pipe puzzle in the northeastern portion of the map. To complete this puzzle you will have to collect a total of 4 items which are found throughout the sewers then use them in the appropriate order at the right pipes. A more detailed explanation of this can be found below.
Hand: This is one of the items required to complete the Pipe Puzzle in this zone.
Ring: Another one of the items required to complete the Pipe Puzzle in this zone. There will be a mural on the wall that you're able to click, doing so will give you The Lover's Ring which is a quest item.
Quallo: Quallo is the only friendly NPC you can speak to in this area. You'll have to defeat the friendly Carrion Crawler next to him for the Blood of Quallo's Friend to solve the Pipe Puzzle.
Shaman's Staff: In this area you'll find a pack of Kobolds, the Kobold Shaman will drop a Shaman's Staff as well as the Circlet of Lost Souls.
Pipe Puzzle: This puzzle is part of the Solve The Riddle In The Sewers Side Quest and will require you to collect various items throughout the zone to complete it. Follow the link I provided for a detailed walkthrough about this quest. Alternatively, if you'd prefer to watch a video walkthrough instead check out my BG2 - Solve The Riddle In The Sewers Youtube Walkthrough.
For those of you who don't need an indepth walkthrough to solve this puzzle, I have included a single picture below which should help save you some time and solve it.