Gather some allies and head down into the caverns beneath Old Ironforge. Enter the Hall of Thanes there and push to the lower vaults.
My Dark Iron is a bit rusty, but I can make out a name--one Durgen Dirgehammer. It's likely that he's the commander. Take his head and deliver it to King Magni Bronzebeard.
Oh, and when you do, tell him his old friend Farsen sends his regards!
