The Last Holdouts

The Last Holdouts is a side-quest in act III of Dragon Age 2 given by the Templar Ser Mettin

Acquisition
You must have favored in dialogue with Meredith in the beginning of act III. You must have completed some quests before it will be acquirable. Head to The Hanged Man and a bartender there will tell you that some men are waiting for you in the back. retrieve the quest here.

Walkthrough
Head towards Darktown and head into the Sanctuary and kill off all the Blood Mages in the area. You don't have to read all the letters lying around to continue the mission. Once they're all killed off head back towards your Estate and read the letter from Ser Mettin. Head towards Sundermount past the Caverns and into the Graveyard, head towards the Marker where the Blood Mage Refuge hideout will be. Ser Mettin and Ser Agatha will assist you in killing off the blood mages in this area. Mettin will then wish to kill the others assisting the mages there as well, which Agatha disagrees to. You have the option whether to allow Mettin to continue or Fight him. Templars will then show up and you'll have to fight Mettin with the aid of Agatha and several templars.

After the fight unlock the Complex-Chest for the The Vague Blade

Result
The blood mages were dealt on behalf of the templars, who promise to provide future assistance.

Extras
The Vague Blade