Haer'Dalis Companion Quest - Baldur's Gate 2
Haer'Dalis' Companion Quest is called Rescue Raelis and Haer'Dalis from the Planar Prison and it's a quest I already have a guide for on my site. For more information about the quest I recommend you follow the link I provided. This quest will begin immediately after you turn in the Free Haer'Dalis from Imprisonment by Mekrath side quest which is how you get Haer'Dalis in the first place.
Essentially what happens is after you turn in the Free Haer'Dalis quest Raelis will ask for your help with another one. This one involves you protecting them as they open a portal to another dimension and this quest will act as Haer'Dalis' Companion Quest. At the start of this quest Haer'Dalis will be abducted and you'll be tasked with entering the Planar Portal to save him.
Technically you can recruit Haer'Dalis into your party and then never turn in the Free Haer'Dalis side quest which will allow you to keep him in your party indefinitely without ever having to do his companion quest. However, I will say this, Haer'Dalis' companion quest is extremely important because it's one of the only legitimate ways to get Paws of the Cheetah in BG2.
Once inside the Planar Prison you'll find Haer'Dalis in the northwestern corner. In order to free him from this zone you'll need to defeat the Warden (pictured above) who is on the platform that leads up to Haer'Dalis. As previously stated I have a full write up of how to solve this quest on my Rescue Raelis and Haer'Dalis from the Planar Prison quest walkthrough page.
If you'd like a complete overview of the Planar Prison as a zone then you should check out my Planar Prison Overview page. Both of these pages will provide you all the information you need for navigating this map and completing the side quest. Once you approach Raelis and Haer'Dalis you'll get some dialogue with them after which you'll be teleported outside of the Planar Prison and the quest will be concluded.
Note: All loot inside the Planar Prison will be gone and you'll never be allowed to enter it again once this quest is complete. For this reason it's recommended you be thorough and loot everything you want!
Back at the Five Flagons Inn you'll have a bit more dialogue with Haer'Dalis and Raelis. If you're playing a Bard as your main character Raelis will offer the Five Flagons Inn basement area to you as the Bard Stronghold. For more information about the Bard Stronghold Quests follow the link provided.