Free Hendak and the Slaves - Baldur's Gate 2 Side Quest
Free Hendak and the Slaves is a side quest that begins by speaking with Hendak in the back rooms of the Copper Coronet. To get into the back rooms you'll first have to speak with Lehtinan who is the current proprietor of the bar; ask him for ways you can get rid of your excess coin and about companionship and he'll allow you access to the back.
The moment you enter the hallway with all of the slave jail cells though the guards of this fine establishment will become hostile -- so keep that in mind. Approaching the jail cell that contains Hendak will begin the dialogue for this side quest. In order to complete this side quest you'll have to do 2 main things, I will list both of them below then explain each in more detail.
1. Defeat the Beastmaster in the Copper Coronet & use the Beastmaster Key he drops to open Hendak's cell
2. Travel to the Slaver Stockade through the Slums Sewers and free the other slaves that are here
You can find the Beastmaster that we need to eliminate in the southeastern most portion of the Copper Coronet. When you first engage him he'll open all of the cages near him freeing all of the animals and monsters; this will result in you fighting him + all of the animals. I wouldn't worry too much though as it's not a very difficult fight.
Once you have the Beastmaster's Key you can use it to free Hendak from his prison cell. Hendak will make a B-Line straight for Lehtinan at the front counter and kill him after which he'll ask you to find the rest of the slaves being kept nearby in the Slaver Stockade.
In order to reach the Slaver Stockade you'll need to travel through the Slums Sewers, this will allow you to enter into the back entrance of the Slaver Stockades. All you need to do here is free a single prisoner (pictured below) then return to Hendak and speak with him to complete the quest.
There are a few more slaves in the Slaver Stockade that you can free though for a total of 5000 EXP in addition to probably a dozen different magical items for you to collect.