Iou

The iou was a quest item that appeared in the Death Plateau quest. Harold gave it to the player in lieu of payment for a gambling debt he owed from a dice rolling game.

When the player inspected the back of the IOU, they discover that it is really the combination to the equipment room in Burthorpe Castle.

You must have had this item in your inventory when speaking with Denulth to complete the quest. If lost, Harold wrote you another one.