Treasure Bags are consumable items obtained in Expert Mode as a reward for defeating bosses. Its contents depend on the boss it comes from. It is opened by either right-clicking it inside the inventory or using it while it is in your hand. In multiplayer, each player that is online at the end of the battle receives one Treasure Bag each. Treasure bags are client-side and will despawn if you exit the world.
Each Treasure Bag contains the boss's usual drops from normal mode, along with at least one Expert Mode-exclusive item (pink-highlighted items below).