Baldur's Gate II Walkthrough: The Underdark
After you rescue Imoen from the Asylum and go through Sahuagin City in "Baldur's Gate II," you'll wind up in the Underdark. You must find your way through the Drow city of Ust'Natha in order to return to the mainland.
Baldur's Gate II Walkthrough - A Mage in the Underdark
You arrive in the Underdark at the southwest corner of the map. To the east is a camp of Duergar. Talk to them to learn of an imprisoned mage in the area and purchase a Freedom scroll.
There's a clearing to the north with three gates nearby: earth, air, and fire. Elementals will spawn from each of these gates and attack you. Since they'll respawn about a dozen times after you kill them, the gates are a good way to gain some experience. Cast Freedom in the middle of the clearing to release the imprisoned mage, Vithal. He can traverse the Elemental gates, but he needs you to find the Book of Rituals first.
Head north to the Svirfneblin settlement and talk to the Innkeeper. You'll discover that his son, Bedlen, is missing; you can find him in a prison globe to the south. Activate each window to release a prisoner; you will need to fight all of them except for Bedlen. Return to the Innkeeper for the Book of Rituals and some Bracers of Defence AC 4.
Bring the Book of Rituals back to Vithal and help him defeat the Elementals at each gate. When you're done, Vithal will give you a Rod of Absorption. If you ask for a greater reward, you'll have to fight him, which can be tough. You can loot some ninth-level scrolls from his body, along with the Skull of Death, which allows you to cast the Death spell.
Baldur's Gate II Walkthrough - Underdark Map
Baldur's Gate II Walkthrough - Entering Ust'Natha
Return to the Svirfneblin settlement and talk to Goldander Blackenrock, whom you'll find in the first room. He will tell you about a horrible creature that the Svirfneblin discovered when they dug too deep. Defeat the creature in the pit to the east and seal the hole with Blackenrock's Stoneshape Scroll. He'll reward you with the mace Skullcrusher +3 and a light gem.
Once you have the light gem, you can enter the lair of Adalon, a silver dragon, located to the northeast. A nearby exit leads to the Drow city of Ust'Natha, which you can't use because you're not a Drow. Adalon will cast a Drow Illusion on your party so you can move through the city.
In order to leave the Underdark, you must save Adalon's eggs from the Matron Mother in Ust'Natha, who plans to use them to summon a demon. Once you've recovered the eggs, return to Adalon. She'll open an exit to the surface from the Kuo-Toa Dungeon. The dungeon is located to the east in the Underdark, but the exit is heavily guarded until you've completed Adalon's quest.
The Underdark also has three dungeons: the Kuo-Toa dungeon to the east and the Mind Flayer and Beholder dungeons to the southwest. You can explore these dangerous areas now, or after you pick up the relevant quests in Ust'Natha.