Key of Night

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

Spawning the Corrupt/Crimson Mimic consumes both the Key of Night and the chest.

Trivia

 * This and the Key of Light may be a reference to Kingdom Hearts.

History
Clé de nuit 밤의 열쇠 Ключ ночи