You have to talk to Sir Tenshin in Shoushi to be flagged to enter this dungeon. Enter the dungeon using the trap door in front of the fireplace in the farm house. Use the candle on the wall to reveal a passageway behind the fireplace in room you portal into. Run down to passageway until you come to two rooms with 2 Swamp Rats. In the room on the left you will find an unlocked Storage Chest with a Folded Guard's Uniform and A Folded Note. The uniform is what Sir Tenshin wants as evidence.