Treasure Bag

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 deals damage to the boss receives one Treasure Bag upon completion. Treasure Bags are client-side and will despawn upon exiting the world.

Each Treasure Bag contains the boss's usual drops from normal mode, along with one Expert mode-exclusive item (pink-highlighted items below) – with the exception of the Lunatic Cultist, Dark Mage, Betsy, and Ogre, who currently do not provide an Expert mode-exclusive item. Hardmode bosses' Treasure Bags additionally have a 5% chance to contain one random set of developer items.

Expert mode-exclusive items all have the rarity, with the exception of the Suspicious Looking Tentacle and 0x33's Aviators.

Internal Rarity
Each Treasure Bag has an internal rarity, despite being expert items, which always have a rarity.

Developer items
In addition to the above, all Treasure Bags from Hardmode bosses have a 5% 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.

Trivia

 * The Moon Lord's Treasure Bag is the only one to include both two Expert mode items and no money.
 * Betsy's Treasure Bag is the only (obtainable) bag that does not have an Expert mode item. It is also (along with the Moon Lord's) one of the only ones to contain no money, replacing coins with Defender Medals.
 * The Lunatic Cultist, Ogre, and Dark Mage have sprites for a Treasure Bag, even though they do not drop one.
 * Unlike the Lunatic Cultist's Treasure Bag, the Ogre and Dark Mage do not have data assigned to their Treasure Bags, as attempting to hack in either one will cause them to disappear from the player's inventory instantly.

History
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