Underdark Overview - Baldur's Gate 2
The Underdark is the dungeon that you'll be spending your time in during the entirety of Chapter 5 in the main quest chain. Aside from the main quest there are numerous side quests that take place in this zone such as the entire Ust Natha chain that starts with speaking with Adalon in her lair.
You can also find numerous side areas connected to this map. To the west you can find the Western Kuo-toan Tunnels, to the east you'll find the Mind Flayer infested Eastern Tunnels and to the south you'll find the Beholder Southern Tunnels. I'd recommend you spend time clearing out all 3 of these areas before starting the Ust Natha quest chain since almost all of the Ust Natha chain is timed and you won't have a lot of time for dillydallying while the quests are active.
Vendor: This is one of the 2 Svirfneblin vendors in the Underdark; he sells a variety of equipment and scrolls that you might be interested in. I strongly recommend you buy a Freedom Scroll from this vendor and use it while in the Underdark, this will cause Vithal the mage to appear.
Air Portal: At this location you will find a portal that will summon 11 Air Elementals that you can farm for EXP. This location is also involved in the A Mage's Proposal Side Quest that begins by speaking with Vithal.
Earth Portal: At this location you will find a portal that will summon 11 Earth Elementals that you can farm for EXP. This location is also involved in the A Mage's Proposal Side Quest that begins by speaking with Vithal.
Fire Portal: At this location you will find a portal that will summon 11 Fire Elementals that you can farm for EXP. This location is also involved in the A Mage's Proposal Side Quest that begins by speaking with Vithal.
Vithal: In order to make Vithal appear on this map you'll have to use a Freedom Scroll or cast the spell yourself on one of your Priests who have it. After using Freedom you can find Vithal at the intersection between the Elemental Portals, an exact location is shown on my map above. Vithal will give players the A Mage's Proposal Side Quest which requires you to visit each of the 3 portals and protect Vithal as he steals items from the other worlds.
Soul Prison: Here you will find a spherical prison that you can interact with and release prisoners from. There are a total of 6 prisoners you can release from this prison all of which will drop a unique magical item and sometimes some high level scrolls. I will talk about all of them more indepth on my Victims of Imprisonment Side Quest page.
Svirfneblin Inn: As the name suggests this is the Svirfneblin Inn with your normal inn services. The Innkeeper here will sell you a book needed for the A Mage's Proposal Side Quest and in the back of the Inn you'll find Therndle Daglefodd who starts the Svirfneblin Son Missing side quest.
Goldander: Goldander Blackenrock is the leader of the Svirfneblin settlement and he is one of the NPCs you'll have to speak to in order to advance the main story during Chapter 5. When you first talk to Goldander he'll ask you to help him and his village by taking out a powerful demon and sealing the pit that him and his homies dug. You can find the demon he is talking about to the northeast, it's marked Demon Tunnel on my map.
Demon Tunnel: This room will be locked until you accept the task from Goldander to kill the powerful demon & seal the tunnel. When you click on the ladder leading down into the pit you'll summon a Balor which is a very powerful demon. It can only be damaged with magical weapons that are +3 or better. Once it's defeated use the Stoneshape Scroll to collapse the tunnel then return to Goldander.
Adalon's Lair: In order to enter this area you will first need the Light Gem from Goldander in the Svirfneblin Village. A silver dragon by the name of Adalon can be found in this cave and you'll need to speak with her to begin the Recover the Silver Dragon's Eggs quest. She will cast a Drow Illusion on you which will allow you access to Ust Natha and the entire Inside the Drow City quest chain that takes place here.