The Warden can get this quest after the corpse of Ser Friden is found in the Run-Down Back Street in Denerim. After his journal is picked up, the quest will be activated and you can travel to the Deserted Building.
Go to the Deserted Building in Denerim, some mercenaries and blood mages will attack. As usual, the mages are the biggest threat so kill them first. There are also some deadly traps which should be disarmed by a rogue. In the last room the leader of the blood mages will be waiting. After he is killed, loot his body to find a key to his bedroom where you can find the Magister's Shield amulet. (Rogues can also pick the bedroom lock.)
This encounter is one of the more challenging fights in the game; do not attempt this without a top notch group. There is an anteroom prior to the entering the main hall. Any traps set here will persist after the dialogue with the blood mages, which means that setting multiple traps here to bleed off HP from the mages and mercenaries works well. Killing off the non-elite blood mage early, to prevent him from healing his companions, is highly recommended. The templar ability Holy Smite is effective, as are the mage spells Blizzard, Tempest, and Storm of the Century, especially if Paralysis Explosion is used first to prevent targets from escaping early. Mana Clash is also very effective. Other way, when the fight start immediately interrupt him alternately, and for the other mage just send 1 warrior to kill him, ignore the other as you heal to kill the mage.