This quest can be completed with lockpick but for those without the skill they will need to retrieve a number of keys from the dungeon to gain access to the shield.
Walk Through
First make your way to the Bandit Castle (66.5N 50.0E) in the mountains north east of Holtburg. Route: In Arwic, take the Mountain Shortcut portal at 35.0N 56.0E. Run east and take the Bandit Castle portal at 42.4N 60.5E.
Once at Bandit Castle, enter the Bandit Castle Prison by taking a ramp into the center room. There's a ramp downward to the portal room.
From the drop point go straight ahead and right at the split. You will come to a large open room with creatures locked in cells and a pit in the center.
Jump down the pit and exit to the North East (East door, go left.)
Go left into the main room and take the stairs in the south east corner. Use the Bridge Access Key on the next door to access the bridge leading to a room with 3 Revenants and a chest.