Key of Night

Placing this inside an empty chest will turn it into either a Corrupt Mimic if the world generated with The Corruption, or Crimson Mimic if the world generated The Crimson. The key does not need to be used in the Underground Corruption or Underground Crimson. The key's counter part is the Key of Light.

Hardmode must be enabled for it to work, and key must be only item remaining in chest.

History
Clé de la Nuit