Underdark Western Tunnels Overview - Baldur's Gate 2
The Western Tunnels are commonly referred to as the Kuo-toan Tunnels because you'll be fighting a lot of Kuo-toan enemies here along with a Kuo-toan Prince. Aside from the Kuo-toan Prince the other 2 important landmarks in this area are the Demogorgon Statue as well as the route leading out of the Underdark.
You can find the Demogorgon Statue in the southwest portion of this area; use almost any summon spell in the game at the base of the statue to make 5 Demon Knights appear. As for the route leading out of the Underdark, at first you will find it guarded by a group of Drow who are hostile to you unless you have the Drow Illusion from Adalon.
If you have the Drow Illusion from Adalon you'll be able to talk to the Drow and even possibly convince them to open the door and let you leave. Having high Charisma (17 or more) and Viconia in your party will make it significantly easier to convince the Drow to let you pass. Alternatively, you'll have to return the Dragon Eggs to Adalon and complete that quest to gain safe passage.
Tadpole Room: Inside this room you can grab Corrupted Tadpoles from the pool of water around the room's edges. These Tadpoles are used in the Kuo-toan Prince's room in the northeastern portion of this area; use them on the pool of water in this room to prevent the Kuo-toan Prince from regenerating his health.
Demogorgon Statue: At this location you'll want to summon a monster or animal at the base of the statue, this will offer it up as a "sacrifice" and spawn 5 Demon Knights. Successfully defeating the Demon Knights will earn you 82k EXP along with four Two-Handed Swords +1, Armor of the Hart +3, Girdle of Frost Giant Strength and Soul Reaver +4. Aside from being an amazing belt the Girdle of Frost Giant Strength is also used in the Cromwell's Blacksmith side quest to create Crom Faeyr.
Kuo-toan Prince: Here you'll find a Kuo-toan Prince enemy who regenerates his health at an insanely fast rate. To counteract this you'll want to visit the Tadpole Room earlier in this dungeon and grab some Corrupted Tadpoles from the water surrounding the room. Place these Corrupted Tadpoles into the pool of water in the Prince's room to disable his health regeneration. Your reward for defeating the prince will be Bracers of Blinding Strike, Gems and 4500 Gold along with Kuo-toa's Blood which can be used in the Inside the Drow City side quest in Ust Natha.