Key to the City

The Key to the City is the master key to the city of Stratholme. It was required to open the Eastwall Gate that serves as the entrance to the Scourge-controlled eastern districts of Stratholme, as well as the gate between King's Square and the Festival Lane (linking the western and eastern districts) and the gate into the Gauntlet that begins the 45-minute countdown to reach Baron Rivendare.

Source
This item drops from Magistrate Barthilas in Stratholme.