Key of Night

The Key of Night is a craftable item that spawns a Corrupt Mimic or a Crimson Mimic when placed in any slot of any empty Chest, depending on whether the world generated with The Corruption or The Crimson, respectively. The world must be in Hardmode and the only item inside the Chest must be a single Key of Night.

The respective Mimic is spawned as soon as the Chest is closed, destroying both the key and the chest.