The Black Key is used for the four Black treasure chests that appear throughout Paper Mario: The Thousand-Year Door. These chests will have a Black Chest Demon that will place a "curse" on Mario that gives him a new Paper ability to use. The keys can be easy to find and usually not far away from the actual treasure chest.
It is unknown as to why these keys exist as Shadow Queen's curse was suppose to trap them for eternity.
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