The Crown of Thieves
After communicating with Zarek by means of your awesome Lightbringer skills, he says that he wants you to complete his failed mission. To retrieve a crown.
He tells you to bring Zannon along with you so go do as he says.
Next, since there is a party limit of 3, one of your people have to leave. The choice is up to you and how your party is comprised. Coren is a warrior by base stats, Corra is a Wizard, & Zannon is a Marksman. So if you have been giving Coren marksman equipment (like I did), it is a better strip Corren's equipment and go with Zannon since she is a Marksman by base stats. Remember to equip her with good equipment as well.
Zannon mentions that you ought to buy a few lockpicks since you will come across many unlockable chests in the place you are going. Buy them from Shady Dan. The pedestal beside him also teaches you how to use lockpicks. But the most notable item here is the Green Sack beside him. It gives you an extra 15 storage (it gives 16 but since the sack itself takes 1 storage, it gives 15). Take it to be able to haul more stuff.
Head to the quest marker in the sewers to find the trapdoor to the lower sewers.
Enter the trapdoor.
On the short, narrow path, you will come across a big slime monster. Nothing dangerous and could be dispatched off easily.
When you reach the end, you will find the ladder leading to the treasury. You will find a tax collector. You have an option of give in to his extortion of 2500 gold, reduce it to 1500 gold if you have 10 intellect, or just outright kill him. I went for just killing him since he is corrupt and you will alert enemies when you open chests anyway.
Head to the quest marker while looting chests and killing corrupt tax collectors.
Here you will find Edward. He will offer another way to solve this conflict.
His plan involves swapping the real crown with a replica which is stored in the other side of the treasury. If you accept it, he will give you the key and you can continue.
Head over to the other crown and take it.
After arranging the swap, he gives you the real crown and sends you off.
Bring the crown back to Zarek and he tells you that your heart was bought alongside many others by a necromancer.