OKAGE: Shadow King

The Forgotten Sword can be found in the World Library dungeon. You must first progress far enough to acquire the Amethyst key. After, head to the east wing and go through the door at the north west corner of the map. Follow the path and you should arrive at a small, separate room in the central library. In the room you should find a chest containing the Forgotten Sword. This sword can reveal passageways in previous areas and dungeons which contain some nice items. None of items are necessary to beat the game or obtain your Platinum in any way though so don't feel the need to backtrack unless you want to.