Act of Mercy - The Templars
The apostates will be calm enough to talk now. There are a few options here, and they don’t necessarily end as you’d expect.
The first is to simply insist that the mages come back out with you. Your mages won’t like that, while Carver and Fenris will be happy. You can also agree to kill Thrask, or just tell the templars that they’re dead. Note that the best option is to decide to lie. Just about everyone, except Fenris and Carver, will like you for it, and it’s one of the only ways to end the mission without a bloodbath.
You’ll need to carve through some more corpses on your way back out. It’s just a few more speed bumps, so keep rolling.
If you go outside, you’ll find that Ser Thrask has company. Ser Kerras will show up with his war party and he wants to execute them all.
If you brought the mages with you, then your hand is forced. You can either let them be detained by him, or fight Kerras. There is no middle ground.
If you picked the other options, you will have a few different choices. Agreeing to help the mages in either way means that they won’t be with you, and you can choose to have Varric spin a wild tale that convinces the templars to just leave on their own (with the possibility of Hawke being able to tell a similar lie). Any other choice will just lead to a fight though.
If you choose to fight the templars, you will need to have your party split up. Have half attack the archers on one side and half tie up the fighters. Thrask will fight on your side and should tie up Kerras all on his own. Take out the support quickly, and then watch for the reinforcements. The archers aren’t a huge threat, but the templar hunter can cause some pain. He has a powerful backstab attack, so have a few people gang up on him to quickly kill him.
Kerras is actually quite easy to kill. He just has a lot of health. Without any support, and with Thrask’s help, he’ll die easily.
Finish the quest with Thrask, regardless of your choice, and then pick up the loot that’s left behind.