Black chest

The four Black Chests each contain one of the heros that helped to defeat the Shadow Queen 1000 years ago in Paper Mario: The Thousand Year Door. The heros are now Black Chest Demons and curse Mario when opened. The curses are actually the most helpful power ups in the game. The Chests are in Hooktail Castle, Rougeport Sewers, Twilight Town, and Keeelhaul Key. You open the chests with Black Keys.