You've discovered a rusting key on the corpse of a fresh kill. It has the symbol of the Deathfist Orcs on it, but what could if unlock? If it could perhaps open a secret door of the Deathfist Citadel then it may provide you an opportunity to catch the orcs inside unaware.
The objective of this one-time instance is just to slay everything in sight. Work your way up from the Sewers until you reach the Emperor's Terrace. When you've sated your lust for orcish blood, you can leave the way you came or flee back to Zek from the main gain.