Free Haer'Dalis from Imprisonment by Mekrath - BG2 Side Quest
The Free Haer'Dalis from Imprisonment side quest begins by speaking with Raelis Shai in the basement of the Five Flagons Inn. When you first enter the basement you'll witness a performance by the Sigil Troupe after which they'll invite any adventurers in the audience backstage (this means you).
Raelis Shai is the current leader of the Sigil Troupe and will give you this side quest when you speak to them backstage. You're tasked with finding Haer'Dalis and a Portal Gem from the sewers in the Temple District. Both of these things are found in Mekrath's Laboratory/Hideout which is one of the many different zones in the Temple Sewers.
In order to reach the Temple Sewers you'll need to first travel to the Temple District then go down one of the many different hatches into the sewers. There are numerous quests that take place down here, for a walkthrough of them all I recommend you check out my Temple District Overview.
You can find the entrance to Mekrath's Hideout in the northern most portion of the Temple District Sewers map. Use my screenshot above if you need help finding where Mekrath's Hideout is inside the sewers. Inside Mekrath's Hideout you'll find Haer'Dalis but he'll currently be mind controlled by Mekrath. You'll have to either defeat or help Mekrath in order to free Haer'Dalis from the mind control.
To help Mekrath tell him that you seek adventure, this dialogue option will lead to him asking you for an errand.
If you choose to help Mekrath then what you'll have to do is return to the previous sewers area and go to the bottom right hand corner of the map which is where you'll find the Imp that stole Mekrath's Mirror. Take out the Imp, loot the mirror then return it to Mekrath and he'll free Haer'Dalis as well as reward you with 18,750 EXP. You can still attack him if you provoke him during the dialogue after turning in the mirror allowing you to get EXP for both the mirror and killing Mekrath.
Alternatively, you can force attack Mekrath and kill him to free Haer'Dalis as well. Doing it this way will cause you to lose out on the almost 20k EXP for fetching the mirror though. Once Mekrath is dead or gone you'll be able to speak with Haer'Dalis and you'll also be able to collect the Portal Gem which we also need for this quest.
You can find the Portal Gem in the temple room in the northeastern most portion of this area, a picture of it is provided below. Make sure you grab this gem before leaving since it's required to complete this quest.
Now that we're done with Mekrath's Hideout you'll want to return to the Five Flagons Inn in the Bridge District. Just like before you'll want to talk with Raelis in the basement of this inn. While turning this quest in there will be a scene during which a Planar Portal will be opened and you'll have to fight some easy enemies that come out of it. Unless you refuse to help the Sigil Troupe this is all automatic and you will be thrown into this event.
During this event the Sigil Troupe will be kidnapped and you'll have to go through the portal into the Planar Prison to help them. This will begin the next quest in the chain - it's also highly recommended you do this quest as there is quite a bit of amazing loot inside of the Planar Prison!