Speak to Lehtinan in the Copper Coronet in the Athkatla Slums and he will allow you to enter the back rooms of the inn for the special entertainment. Enter the northeastern section of the building, which is off-limits for visitors.
You will be warned by the guards to leave as quickly as possible or they will otherwise attack you. After beaten the guards, Hendak, the slave, will beg you to free him and the other slaves.
From this point, you have the option of reporting Hendak's escape plan back to Lehtinan, who will give you the Beastmaster Key and ask you to kill Hendak instead (Aerie'll turn hostile but Keldorn will not, surprise), it's recommended to kill Beastmaster for his loots before speaking to Lehtinan however, as the Beastmaster will disappear once the key is in your possession. You lose 1 reputation following this route but Bernard still opens his bonus shop list under Lehtinan's order.
Return to the slave prisons, free the boy and girl in the side cells first or they both disappear after the later stage and you miss the chance to get the XP for freeing them. Once Hendak is out of his cell, all the gladiators will break loose and they attack all guards which they come across.
Follow Hendak through the inn until he confronts Lehtinan. Watch or help him win the duel but don't aid Lehtinan or you'll end up killing both, though Bernard's extra items are still unlocked due to a bug. After the fight Hendak will ask your party to free more slaves who are being held in the Slavers Stockade in the slums.
There is a secret stairwell near the gladiator prison which will lead to the sewers. Pass through the sewers to the Slavers Stockade.
When you leave the sewers, you will be confronted by Haegan. Kill him and take his key, which is needed to open the cells.
Use the key to open the door in the northeast wall, defeat the trolls, and free the girl.
There are two more cells ahead which hold prisoners. Free them.
You can either keep killing slavers and leave through the front door or return to the entrance to the sewers. If you leave through the front door, a slaver guard outside will fight you if you pick the lock on the door. You may have to follow him back inside where he thinks his comrades are, if you intend to kill him too.
Return to Hendak in the Copper Coronet. You will be rewarded with 38000 XP for each party member, +1 reputation, 3900 GP and Kondar.