Following the Qun Dragon Age II Guide
Last update: 11 May 2016
Received from: automatically after completing All That Remains [Hightown - Hawke Estate]
Step 1: Meeting with Viscount Dumar
You should receive this mission automatically after completing the Offered and Lost main quest, so after eliminating the serial killer responsible for killing Leanra. You will be informed about receiving an important letter from the Viscount. Stay in [Hightown - Hawke Estate] and read the Following the Qun letter by the desk #1 (M14, 12). From it #2 you will learn that the Viscount wants to discuss something important in private with you.
Now you can head to [Hightown - Viscount's Keep] by daylight. After getting there, go into Viscount Dumar's office (M9, 7) and click on him #1 to initiate a conversation. The Viscount will tell you that his son Seamus decided to join the Qun #2. Dumar will ask you to make sure whether his decision was made out of free will.
Step 2: Heading out to meet with Arishok
You can now leave the keep and head to [Docks] during daytime. Curiously, you won't be taken directly there. Explore [Winding Alley] #1 (M73, 1). Look out for thugs hiding nearby. Before ordering an attack, try to disarm the trap between you and the enemy forces #2.
During the fight you should first take care of quickly eliminating the Thug Leader #1. Only after his death should you move to attacking the normal Thugs. At the same time be ready to quickly eliminate the group of Mercenaries and Mercenary Archers, who will soon start appearing in the area #2. After the fight I'd definitely recommend searching the Thug Leader's corpse, thanks to which you will gain the Helm of Enasalin.
Now you can use one of the two exits from this location (M73, 2). After landing in the Docks, head to where Arishok is #1 (M19, 8). From the Qunari leader you will learn that Seamus really did join the Qun out of his free will and he headed out for the Chantry to speak with his father #2. You will be able to return to [Hightown - Viscount's Keep] to tell Viscount Dumar about it, but it's not obligatory.
Step 3: Going to the Chantry
Head to [Hightown - Chantry] during the night #1. After getting there (M8, 3) a cutscene will play #2, during which you will learn that Seamus has been killed as part of a plan of Mother Petrice.
The conversation with Mother Petrice can have two courses. If during Offered and Lost you killed Ser Varnell and the other fanatics, Petrice will want to frame you for the death of Seamus. As a result, be ready for a rather easy fight with a bunch of enemies. Calmly wait for the enemies to come (M8, 4) and start off by killing the Mob Leader #1, as he's able to influence the others. Afterwards move on to eliminating the Righteous Mob #2. You will have to eliminate two groups of very weak enemies in total (the second will appear as you go down).
If during Offered and Lost you joined forces with Ser Varnell and the other fanatics, sister Petrice's attitude will be much more friendly. At the end of the conversation you would have to choose the lower dialogue option #1, as choosing the upper one would mean defying her actions and have the same result as killing Ser Varnell (fighting a bunch of people). Joining forces with Petrice would mean having to eliminate two groups of Qunari. Wait until the enemies come to you (M8, 4) and during the fight look for stronger Ashaads #2. There will be two groups to eliminate in total (the second will appear as you go down).
If you didn't join forces with Petrice, you will see a cutscene during which she will be executed #1 by one of the Qunari warriors. One the other side, if you fought alongside her, she will stay alive and a short cutscene with her and Grand Cleric Elthina will play #2.
Step 4: Leaving the Chantry
Irrelevant of Petrice's fate, one last cutscene involving Dumar will play at the end of the quest #1. He will want to say goodbye to his murdered son. Also in this case the choice of dialogues #2 won't play a big part. Soon you will be moved to [Hightown], where you will receive 3 sovereigns and the mission will end. Additionally, if you cooperated with Petrice, you will have a short and irrelevant conversation with her.
The game will add a new main quest to your journal and also update Demands of the Qun, which was on the list from the start of Act II.
Main quest unlocked: To Catch a Thief