A concept called mystery bag in expert mode when you defeat a boss there is a chance you get this what it does it gives you a random treasure of any boss
Terraria 1.3 TREASURE BAG OPENING #1 - YouTube
Items - Tukor the Ungrateful treasure bag, mask, and trophy (let this poor little boss live) | Terraria Community Forums
Farming some treasure bags....I need some yoyos. : r/Terraria
Terraria $3,000 Race, But Treasure Bags Give Random Items... - YouTube
The Lunatic Cultist treasure bag is currently unnobtainable, if it was to be implemented what expert mode drop would you give it? keep in mind it has to be balanced considering the