1. Arx - Prison's entrance
2. Arx - The Vault of Linder Kemm's entrance
There are various ways of starting this quest.
During Mercy Is Power quest you must decide about Wendigo's fate. By sparring her you learn that Linder Kemm serves the God King.
If Paladin DeShelby was rescued during The Execution quest then you can meet her in Prison. She tells you that Linder Kemm isn't what he seems to be and that he has a few secrets.
You should also go near the city gate. There, you can meet a painter. If you use Spirit Vision in that place then you can see a spirit of the painter's father. Speak with him and you learn that Linder Kemm is obsessed with art and he keeps all his pieces in a vault under his mansion.
But before you go to the vault, you must first visit Sewer. In the room with Death Fog you can find secret door that leads to a room with a group of trolls and children. Enter the room. A boy called Big Tomorrah will talk to you. Use Persuasion to avoid a fight.
Failing in the conversation means that you will have to fight with the trolls. Use chests with Death Fog to kill them. Destroying the chests spreads the substance in a specific area. You can teleport the trolls one by one to those places. However, you must be careful and don't block your party from accessing the Unusual Painting located near a troll called Latif. Win the fight and pick up the painting.
If you managed to avoid a fight, you will have two options. You can buy the painting from Big Tomorrah. However, this can cost you a lot because the boy wants you to pay 10,000 gold. You can also steal it.
Take the Unusual Painting to Linder Kemm's mansion. A lever can be found in the garden. Use it to open an entrance to the vault.
However, you must unlock it first. You need 4 Thievery points to open it.
Warning: Speak with Cat The Appraiser before you enter the vault. She orders you to find three items that are located in the vault (see Old Means Gold quest).
The vault is patrolled by living statues - Lucian The Warbringer. They are rather slow so you can avoid them. If you want to fight with them use Source Vampirism ability to defeat the statues. But you need to weaken them first. You can recognize that moment with ease. The statues start glowing blue and you can notice a status that informs you about them being inactive.
Reach the opposite side of the vault. There, you can find closed door and an incomplete triptych. Use Spirit Vision and speak with a spirit of a Dwarf.
Then, you must interact with the empty space in the triptych and place the Unusual Painting. This opens the door. You reach something that resembles a treasury. Here, you can find another closed door. Interact with Tome Of The God King book to open it.
Before you move forward, you should take Responsibility painting from a wall.
You are now in a room with an altar. Have a short conversation with a statue. After that you must read the plate located in front of the altar. It says: "Only God King knows the weight of Responsibility". Place the Responsibility painting on the altar. This unlocks another hatch.
You reach a room with Lord Arhu. This starts Finding Lord Arhu quest.
You must free him. Use Spirit Vision. Three ghosts, that hold Arhu, will appear. Speak with each one of them and absorb their energy.
Save Lord Arhu and defend yourself from Black Ring. Linder Kemm himself will also join the fight. You must kill him if you want to complete the quest.
Reward: 827,900 experience points