Blackpowder Courtesy - p. 2 Dragon Age II Guide
Last update: 11 May 2016
Step 4: Exploring the Side Alley
Leave the area (M64, 3). Be sure that you have a supply of health potions and a mage capable of healing in the party. Only after wards should you head to [Side Alley] #1. After getting there (M65, 1) you will be able to have a short conversation with Maecon (it's worth to have Aveline in the party, as she will recognize him) #2. As you have probably already noticed, the gas has already been let loose and your task will be to stop it from spreading any further (M65, 2).
Step 5: Closing the gas barrels
The basic difficulty of this part of the quest is that you character will gradually lose health because of the gas. Of course it will be possible to cure them using a mage or potions, but more complications will soon appear. At first quickly locate and pick up the latch #1 (M65, 3). Use it on one of the Gas Barrels found in this location #2 (M65, 4), which will plug it.
Mercenaries and Mercenary Archers will start appearing. Unfortunately you can't ignore them, as their defeat is necessary for another latch to appear. Start attacking the enemies #1. Stay especially cautious during the fight with a clearly stronger Mercenary Commander #2.
As you attack the Mercenaries, pay a bigger that normal attention to the health bars of your party members. Eliminating one of the last enemies will cause another latch to appear #1. Just like before, you have to pick it up fast enough and use it on one of the barrels #2.
The process of plugging in the two other barrels will have to same course, however instead of a Commander, Mercenary Assassins #1 and a Crazed Woman will appear #2. Save your game between the phases if possible, so that you won't have to start from the beginning in case being defeated.
Step 6: Eliminating the Elven Fanatic
After obtaining the last latch, immediately use it on the last barrel as it will end the fights and cause the Elven Fanatic to appear #1 (M65, 5). The conversation with the woman won't last too long #2 and you won't have a big influence on its course.
As only the fight begins, it would be wise to eliminate the Elven Fanatic #1 and moving on to the normal Mercenaries afterwards. Stay cautious, as new enemies might appear in other places of the battlefield #2. Keep fighting until all the enemies die.
Step 7: Returning to Arishok
I'd definitely recommend examining the Elven Fanatic corpse #1, which will let you obtain, inter alia, a Formula and the Brothers' End sword. Use the stairs, open the gate #2 (M65, 6) and head to the exit from this location (M65, 7).
Now you have to once more head to [Docks] during the day. Once again head to the Qunari camp to talk with Arishok #1 (M19, 8). You don't have a big influence on it #2. Note that the quest hasn't come to an end yet and new ones have been unlocked.
Companion quest unlocked: The Long Road
Companion quest unlocked: Dissent
Companion quest unlocked: Mirror Image
Step 8: Reporting to the Viscount
You can now leave the Qunari camp and return to [Hightown - Viscount's Keep] during the day. Head to Viscount Dumar's chamber (M9, 7) and talk with him #1. He will tell you about a new problem - the lost Qunari delegates #2. You will take care of investigating this case in the next main quest, but will receive 3 sovereigns right away.
Main quest unlocked: Offered and Lost