The Black Chest Demons are four entities that 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 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 was only temporary, 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 of the game has the box and gives 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. The Boo hid the Ruby Star in Creepy Steeple, and her box is in a shop's storage room in Twilight Town.
One by one, Mario finds the chests and opens them with the keys, releasing them and letting them "curse" Mario with new powers. While the others appear to be taunting Mario, the Koopa one at least did not appear evil, as he is rehearsing the speech. After they "curse" Mario, they vanish and are neither seen nor mentioned again.
- Chapter Obtained: Prologue
- Power obtained in: Rogueport Sewers
- To obtain power: In Rogueport Sewers, the player must find a room where the first thing that can be seen is a huge yellow block and a barred entrance to another room. To the right of the room is a blue switch; when the player hits it, a set of stairs is revealed. Ascending the stairs, the player should see a doorway and a paper airplane platform. The player must now drop off the side of the platform to grab the black key, and then jump back up the stairs and enter the doorway to receive the first curse.
- To use power: The player must stand on a plane panel (also known as an airplane panel) and press Y. The player then has to press B or land to return to normal.
- Demon: the Goomba heroine.
- Chapter Obtained: 1
- Power obtained in: Hooktail Castle
- To obtain power: For this power, the player must continue past the room with the black chest. There's a door that leads to a room where the black key is. The floor appears to have spikes in it, but they won't pop up until the player grabs the key. The player then has 50 seconds to navigate their way back out of the spike maze before they get crushed, triggering an instant Game Over. The player then opens the chest and is cursed again.
- To use power: The player must press and hold R.
- Demon: the Toad hero.
- Chapter Obtained: 4
- Power obtained in: Twilight Town
- To obtain power: The black key is in a shack just outside Twilight Town in Twilight Trail. Before the player can gain this power, they first need the Shop Key to open the storeroom. A fallen tree blocks the path at the end of Twilight Trail. The player has to use Koops to grab the Shop Key on the other side. The player can now open the storeroom to open the chest and be cursed.
- To use power: The player has to press R and rotate Stick. They can also press B to return to normal.
- Demon: the Boo heroine.
- Chapter Obtained: 5
- Power obtained in: Pirate's Grotto
- To obtain power: In the Pirate's Grotto, the player should see a blown open boat. The player must enter the boat, where they see the evil black chest, waiting to curse them. They must speak to the chest, and then fight the Ember that emerges. Upon defeating the ember, a black key drops, which the player can use to open the chest and receive the fourth and final curse.
- To use power: The player must stand on a boat panel and press Y, then press Y again when they reach another boat panel.
- Demon: the Koopa hero.
- Though the Black Chest Demons are separate characters, the second says that the player fell for their trick a second time, meaning the second one knew of Mario's encounter with the first Black Chest Demon.
- In the section on finding hidden Star Piece panels in game's instruction booklet, the picture depicts Mario spin-jumping with Koops in front of the Rogueport Sewers Black Chest which appears closed. This is a continuity error, as Mario doesn't acquire Koops as a partner nor the Ground Pound ability until some time after he opens that chest.
- The heroes hid all seven stars, the curse was only activated when they no longer had any stars, which is why it seemed that only the last four stars had curses on them.
- The Twilight Town Black Chest Demon initially talks using female mannerisms in the Japanese version until the chest is opened; because such gendered speech is difficult to express, this is only suggested by a heart at the end of the Black Chest Demon's request in the English version. This serves to contextualize why the Black Chest Demon asks, "Did you think a lovely lady would pop out or something?"