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From Terraria Wiki
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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.
Be aware that when you are about to spawn the Corrupt/Crimson Mimic, both the key and chest will be consumed.