The 'Deserted Building' comes available (on the world World Map) after finding the journal of the dead Templar in a back alley, after receiving a quest from the Chantry Board. The building itself consists of several large rooms and is home to numerous Blood Mages, Mercenary Archers, and other enemies. At the end is a boss Blood Mage and several other enemies. After killing him you receive notification of completing the quest.
It is advisable to always kill or incapacitate the mages first, because they wreak havoc with both fireball (which knocks you back and have a heavy Damage over Time) and winter's grasp (which has a high chance of freezing its target).