Shadow Key

The Shadow Key opens Shadow Chests found in the Underworld. It is not consumed upon use, and one Shadow Key can be used limitlessly to open all Shadow Chests. It can be found in Gold Chests in the Dungeon (which themselves require a Golden Key to open).

Multiple Shadow Keys can not be stacked in a single Inventory slot the way Golden Keys can, however only one Shadow Key is ever required to be in a player's inventory in order to unlock unlimited Shadow Chests, unlike Golden Keys which are consumed upon use.