Temple District Sewers Overview - Baldur's Gate 2
The Temple District Sewers are reached by taking one of the trap doors/stairs down from the Temple District; you shouldn't have trouble finding the stairs as they're all marked on the map. In this portion of the sewers the most notable thing you'll find is a potential new companion, Keldorn as well as Helmet of Dumathoin and Cloak of the Sewers.
In the northern portion of this area you'll find one path that leads you to Mekrath's Hideout and another that will lead you into the Mindflayer Tunnels. You can use Open Lock to access Mekrath's Hideout (which is where you'll find Haer'Dalis as a companion) but in order to access the Mindflayer Tunnels you'll need to defeat Tazok in Windspear Hills Dungeon and loot the Sewer Key from his body.
Tarnor: Tarnor is the leader of a group of bandits hanging out in this portion of the sewers. They'll demand 1000G or your life when you approach them. Defeating them all will earn you a slew of magical items such as Full Plate Mail +1, Hangard's Axe +2, Helm of Charm Protection, Helmet of Dumathoin (if you imported it from Siege of Dragonspear), Plate Mail +1, Small Shield +2 (x2) and Wyvern's Tail +2.
Keldorn: Keldorn is one of the potential companions in BG2.
Rakshasa: At this location you'll find a Rakshasa enemy surrounded by numerous Kobolds. The Rakshasa will drop the Cloak of the Sewers when killed which is actually a very impressive cloak.
Roger: Roger is a vendor that will Identify any magical items you have as well as sell you a wide variety of different potions. If you ask Roger about why he is in the sewers he'll offer you an easy side quest that involves going to the southwest portion of this map and killing a 'Sea Troll'. Completion of this quest will reward you with 9.5k EXP.
Mekrath Imp: This Imp will only spawn after you've spoken to Mekrath and agreed to help him retrieve his mirror. Mekrath can be found in Mekrath's Hideout (northern portion of this zone) and helping out Mekrath with this mirror is part of the Free Haer'Dalis from Imprisonment by Mekrath side quest.