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. The templar ability Holy Smite is also effective, as are the mage spells, Blizzard, Tempest, and Storm of the Century. Mana Clash is also very effective.