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.