Black Chest Demon
The Black Chest Demons are four entities Mario encounters during the events of Paper Mario: The Thousand-Year Door. The demons are legendary heroes of Rogueport's past history who are cursed by the Shadow Queen and locked in four Black Chests. They set "curses" on those who opens their chest; however, the curses are actually beneficial to Mario, giving him new paper abilities.
Grifty tells the story of their adventure and how they came to be trapped in chests:
A strong Toad from Petal Meadows, a wise Goomba from Boggly Woods, a scar covered Koopa who traveled the world and a Boo were the four heroes. They learned of the evil Shadow Queen and teamed up to defeat her.
In the great battle they managed to seal away the demon, but just before the fight was completed she cursed the four heroes. If they ever let go of the Crystal Stars, a Black Chest would appear and lock them away for all eternity.
The heroes knew the seal on the Shadow Queen wouldn't last forever, and they wanted the stars available to one who might need them. They made a Magical Map to the stars, and to stop evil from using it, they put it in a box that could only be opened by the pure of heart. (The merchant in the opening scene had the box and gave it to Princess Peach.)
The Toad was exhausted from his journey and collapsed in Petal Meadows. A healer saved him, but knowing his fate, the Toad gave the Diamond Star to the healer and said to hide it where nobody would find it. His box is in Hooktail Castle.
The Goomba knew the Punies were being terrorized by the nearby monsters, so she hollowed out the Great Tree. She gave the Emerald Star to the Punies where they have been keeping it safe for the past thousand years. Her chest is in Rogueport Sewers.
The Koopa went to hide the Sapphire Star in Keelhaul Key, but Cortez came and easily took it for himself. The Koopa was so tired he didn't stand a chance and now his chest resides in the Pirate's Grotto.
One by one, Mario found the chests and opened them with the keys. The Koopa at least did not appear evil, as he was rehearsing the speech. After they "cursed" Mario, they disappeared, never to be seen again, possibly released from the Shadow Queen's curse.