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 one Expert Mode-exclusive item (green-highlighted items below) in addition to their other drops.
In addition to the above, all Treasure Bags have a 3.33% chance to contain a random set of developer items. Obtaining a Treasure Bag in an Expert world, then opening the bag in a Normal world will retain the Treasure Bag's chance to yield developer items.