Lower Tombs Overview - Baldur's Gate 2
The Lower Tombs is a map that you will be visiting a couple of times throughout the game for various side quests as well as during the game's main story. You'll be able to visit the bottom half of this tomb throughout the entire game but the top half of the tomb will only be accessible during story events at certain parts of the game like during Chapter 3 and Chapter 6. Additionally, the loot you can find in the top half will change depending on the story events happening here at the time.
Spider Hive: Inside of this building you'll find Pai'Na who is the hive-master of the spiders that live here. Upon entering you'll be almost immediately attacked, all in all it shouldn't be a difficult fight even for lower level characters. Pai'Na will drop a Black Spider Figurine and you'll find a Pale Green Ioun Stone when searching the container in the center of this room.
Coffins: Each time you visit the Lower Tombs and drain it of Vampires you'll want to visit these 3 coffins and use Wooden Stakes on them. During Chapter 3 you're required to use Wood Stakes on all 3 to advance the story but during Chapter 6 it's completely optional. Each time you stake a Vampire at these coffins you'll earn 9000 EXP.
Pool of Blood: Before Chapter 6 you will find the Mace of Disruption in this pool of blood. You'll have to click on the pool twice to collect it. As previously mentioned, once Chapter 6 begins some of the things in this area change. During Chapter 6 you'll find Drizzt and his gang of adventurers inside this room.
Locked Door: This door is locked until you visit this location during Chapter 3. If you side with Bodhi then you'll be granted access to the northern half of this area as part of her quest line. If you side with the Shadow Thieves then you'll smash through this door to assault the vampire base during the quest chain.