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.
Having any of Blizzard, Tempest, Inferno, or Death Cloud plus being able to access the bedroom prior to the fight can make this battle much easier. Because dialogue ends with the party near the entrance, retreat them immediately into the bedroom. Have warriors choke off the bedroom door and shut the door as soon as possible. All this forces the blood mages to run after the party and keeps them out of line of sight -- the latter meaning the blood mages will not cast any spells. Cast the continuous area of effect spell into the hallway, and try to keep the door shut with spells that can paralyze or otherwise incapacitate the enemy, making them unable to open it. Inside the bedroom, stand off to the side of the door so that you continue to stay out of line of sight of any enemy, and can converge fire on whoever steps in first. Nice spells here are Glyph of Repulsion to keep the enemy knocked back away from the door and into the continuous area of effect spells; or Cone of Cold to paralyze them and create a traffic jam behind them.
The air around this simple silver chain seems to shimmer slightly, and the chain itself feels weighty, as if it were something far more substantial.
+12 defense against missiles
+4% spell resistance
-10% nature resistance Dalish Battery
A spiraling weave of lyrium circles the band of this ring, somehow drawing and storing energy from the very air.
+10% electricity damage