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
Calamity] Farmed the Devourer of Gods for almost 2 hours trying to get the rare drops (2.5% each). Didn't realize you could just get them from the treasure bag : r/Terraria
Don't ask about the loot bag (cultist) : r/Terraria
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
Switch - Expert Mode Bosses Not Dropping Anything | Terraria Community Forums
If event bosses had treasure bags : r/Terraria
We killed the Moonlord on Master Mode and didn't receive the treasure bag :/ : r/Terraria