Deathbringer can be stolen from a Mimic-like creature in the Barheim Passage. In the screen below the gate crystal where the path forks towards the left of the screen, take the lower path to reach a small area. The rare game will look like a treasure pot and it's tucked against a wall and a rock.


Getting the Barheim Key involves completing several hunts and various forays into areas of Dalmasca Estersand. If you're as far along as you say, then it's not really worth going into Barheim unless you're a total complete-freak like I am.