IOU an item

The IOU an item was a Pre-Hardmode crafting material that was dropped by the Siren and Leviathan if Hardmode had not yet begun. It could be crafted into any of the boss's weapons when combined with a Living Shard.

Trivia

 * 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 had been defeated at the time it existed, Living Shards from Plantera were required to turn IOUs into the boss' various weapons.