Rescue Raelis and Haer'Dalis from the Planar Prison - BG2 Side Quest
The Rescue Raelis and Haer'Dalis from the Planar Prison side quest begins immediately after you turn in the previous one, Free Haer'Dalis from Imprisonment by Mekrath (this apparently just isn't Haer'Dalis' week). At the start of this quest Raelis will open up a Planar Portal which summons a bunch of enemies that you're going to have to help deal with.
After dispatching all of the enemies Raelis and her entire group will be kidnapped by bounty hunters. Our task should be pretty obvious at this point, we're going to have to enter the portal and free Raelis and her merry band. It's worth noting that you can take as long as you desire to enter this portal; I would recommend you go upstairs and have a rest before jumping in since this dungeon is difficult.
Upon entering the Planar Prison you're going to be immediately attacked by enemies. After defeating them you'll get some dialogue with Tagget, a dwarf that is stuck in this prison just like you. He'll give you a quick run down of how this place works and what you need to do in order to escape.
Before you do anything in this dungeon let me warn you - turn off AI on your party and try to navigate this place slowly and carefully. This dungeon is very dense in terms of enemies and if you carelessly run into one group of enemies you'll likely agro another. There are also portals on the ground that you can step on that will teleport you to a prison cell with even more enemies. If you're not careful things can spiral out of control very quickly.
I've included a quick list below which will tell you step-by-step exactly how to complete this dungeon. Despite the difficulty of this dungeon, it is actually quite linear and easy to complete.
How to complete the Planar Prison:
1. Travel East upon entering
2. Defeat the Master of Thralls
3. Use the Mastery Orb on the Brazier
4. Defeat the Warden
5. Free Haer'Dalis
Check my Planar Prison World Overview for more info about this zone
Important: Do not run straight north across the area marked 'Portals' on my map above. These portals will teleport characters 1 at a time into a small room with enemies and it will end up splitting up your party and making things a complete mess.
Much like our quick list above says, your first target is going to be the Master of Thralls which you can find in the northeast portion of the Planar Prison. He will drop the Mastery Orb which can be used to free all the Thralls from their mind control. In order to smash the Mastery Orb and free all of the Thralls you'll need to take it to the location marked Brazier on my map above.
When you're ready to challenge the Warden you will want to have 1 character with the Mastery Orb in their inventory standing next to the Brazier and the rest of your team ready to rush the Warden. Once you destroy the Mastery Orb and remaining Thralls will immediately turn on the Warden and attack giving you a nice distraction to make this fight easier.
Once the Warden is dead speak with Raelis and Haer'Dalis in the northwestern portion of the map to complete this quest and be teleported back to Athkatla. If your main character is a Bard and you don't have any other Strongholds currently then Raelis will offer the playhouse to you as your new Bard Stronghold.