Ust Natha Overview - Baldur's Gate 2
Ust Natha is a Drow City that most of Chapter 5 will be taking place within. In order to enter this city safely you'll first have to visit Adalon and begin the Recover the Silver Dragon's Eggs quest. When you accept this quest Adalon will cast a Drow Illusion on you and your party members which will allow you to safely enter the city and do all the quests inside.
Your first time entering Ust Natha under the Drow Illusion will begin the quest Inside the Drow City. There is a lot to this quest and it's a timed quest which means you should put everything else aside until it's finished. Once that quest is completed you'll be given a followup, The Drow Summoning Ritual. At the end of that quest Ust Natha will become hostile to you and you'll lose access to all of the "town" features here. Make sure you are completed with the zone before that happens.
Tank: During the Inside the Drow City quest you'll be approached by a Duergar who will ask you to speak with his master in the tank. The creature inside of the tank will start the side quest Retrieve Qilue's Brain if you agree to help it.
Vendors: This is where you will find all of the vendors in Ust Natha. None of them have unique names that will make them stand out, just talk to each of the Drow standing around this platform and you'll find a couple of vendors including a spell vendor.
Tavern: The Tavern is a location you will be visiting throughout your time in Ust Natha rather frequently. Phaere and Solaufein will ask you to meet them inside on a few occasions. You'll also find 2 battle arenas inside which can earn you gold and EXP. For more information about what's inside here check my Ust Natha Tavern Overview page.
Qilue's Home: Qilue's Home is only accessible during the Retrieve Qilue's Brain side quest. Inside you'll find Qilue along with about a dozen other Drow all of which give a decent amount of EXP. This side quest will begin sometime during the Inside the Drow City quest.
House Jae'llat: You won't gain access to this building until you've finished the Intrigue With Jarlaxle side quest.
Male Fighter: This is where you'll find the Male Fighter's Society, you'll be visiting this building a few times to meet Solaufein during Inside the Drow City.
Deirex's Tower: Shortly after you begin The Drow Summoning Ritual quest you'll be stopped by a man named Visaj who begins a quest that allows you to enter this tower. The quest involving this building is Intrigue With Jarlaxle.
Temple of Lolth: You'll find Adalon's Eggs in this building which you need for Recover the Silver Dragon's Eggs. The only other thing this building is used for is The Drow Summoning Ritual quest which will involve Adalon's Eggs too.