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 must be the only item inside the chest.

When you are about to spawn the Corrupt/Crimson Mimic, remember that both the Key and the Chest will be consumed.

Trivia

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

History
Clé de la Nuit 밤의 열쇠 Ключ ночи