Go to the Deserted Building in Denerim, some mercenaries and blood mages will attack. The mages are 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.
Strategy: There is an ante room prior to the entering the main hall. Any traps set here will persist after the diaglogue with the blood mages. You can set multiple traps here to bleed off HP from the mages and mercenaries. I have found using several spring traps right after the door to slow down the mercenaries until the mages catch up followed by claw spring traps to then large shrapnel traps to maximize damage. Group them tight and maximize the injury. Lay them in a wide path between the enterance and the side door to the right. When the encounter happens, immediately retreat your party back through the anteroom and into the ajacent room where you will wait for the mercenaries and mages to chase after you ... and for the ambush to happen. If you have a mage with fireball or any range attack that covers a large area then throw it peridically into this anteroom to add to the injury. I have found I can kill almost all but the mage leader and his HP is diminished by half by the time all the traps have been set off.