The IOU an item is a Pre-Hardmode crafting material that is dropped by the Siren and Leviathan if Hardmode has not yet begun. It can be crafted into any of the boss's weapons when combined with a Living Shard.
- An "IOU" (abbreviation for "I owe you") is a document stating one's debt to another. In the case of this item, the debt is a drop from The Leviathan.
- Despite the Siren and Leviathan dropping their weapons after the Wall of Flesh has been defeated, Living Shards from Plantera are required to turn IOUs into those weapons.