All That Remains - p. 3 Dragon Age II Guide
Last update: 11 May 2016
Step 5: Defeating Quentin
The fight consists of as much as five parts, but Quentin will become an active participant only after you advance to the last part. Until then he will be hiding inside a protective barrier #1 and won't interfere with your fights. In the first part you will have to face only a couple Corpses and one Shade #2.
If Gascard came to you to Quentin's lair and hasn't been killed by Varric, the course of battle will change a bit. Successfully convincing Gascard to your side will cause him to help you in all five stages #1. A failure will lead him to attacking your party. ABSOLUTELY eliminate him #2 before taking care of the Corpses and Shade, as he uses powerful blood magic.
The second part of the battle is much more demanding. At the start you will have to quickly eliminate Possession of Ninette #1, as the monster has similar abilities to those of a Desire Demon. Next is line should be the Abominations and Shades #2, which shouldn't be really hard considering their short life bars. Put off the Corpses and Skeleton archers for the end.
You will advance to the third stage after killing all the monsters. This time it's also worth to start by attacking the main demon, which is the Possession of Alessa #1. Attack the other monsters in the same order, but note that some Abominations and Shades have extended life bars #2.
The fourth stage will most of make you face Possession of Leandra #1. Only after eliminating it should you move to attacking the other monsters #2. This time even more of them will have extended health bars, so you will have to try harder this time.
As you advance to the last part of the battle, immediately start attacking Quentin, now without the protective barrier around him #1. It's crucial for winning, as you can't let him cast any more spells. After eliminating Quentin, you can move on to the last group of Shades #2.
A cutscene will start, showing the last moments of Leandra #1, who up until now was kept alive only with Quentin's magic. You can depart with her in a couple different ways #2, but it won't have influence on the further events and the relations with your party members.
The game will jump to [Hightown - Hawke Estate]. Here there are two more conversations waiting for you. First you will talk with Gamlen #1, who you can tell about the killer's motives or leave them for yourself. Afterwards you will probably have a short conversation with one of the party members #2. In the end you should be rewarded with 1 sovereign and the Tine dagger and also one main and one companion quest will be unlocked.
Main quest unlocked: Following the Qun
Companion quest unlocked: The Captain's Condolences
If Gascard helped you in the fight with Quentin, you will be able to do one more thing. Head to [Darktown] (M23, 20). You will have the chance to use Fenris' special action to inflict pain to Gascard #1. At the end of the conversation you will have to decide #2 if the blood mage should be left free (upper right dialogue option) or executed (lower right dialogue option). You won't have to fight in the second case, as only a short cutscene picturing Gascard's death will play.