Black Chest Demon

"You dopes fell right into my absolutely brilliant trap! And now you will SUFFER!"

- Black Chest Demon

The Black Chest Demons were four entities Mario encountered during the events of Paper Mario: The Thousand-Year Door. The demons were originally legendary heroes of Rogueport who were cursed by the Shadow Queen and locked in four Black Chests. They set 'curses' on those who opened their chest; however, the curses were actually beneficial to Mario, giving him new paper abilities (This concept was reused from several The Legend of Zelda games).

History
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.

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 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.

Airplane Mode

 * Chapter Obtained: Prologue
 * Power obtained in: Rogueport Sewers
 * To use power: Stand on a plane panel and press . Press or land to return to normal.

Paper Mode

 * Chapter Obtained: 1
 * Power obtained in: Hooktail Castle
 * To use power: Press and hold.

Tube Mode

 * Chapter Obtained: 4
 * Power obtained in: Twilight Town
 * To use power: Press and rotate . Press  to return to normal.

Boat Mode

 * Chapter Obtained: 5
 * Power obtained in: Pirate's Grotto
 * To use power: Stand on a boat panel and press and press  again when you reach another boat panel.

Trivia

 * Though the Black Chest Demons did not appear in Super Paper Mario, Count Bleck's face and the faces of the three Dorguys look almost identical to that of the Black Chest Demons.
 * When the Demon "curses" Mario, it appears to somewhat threaten him, like giving him bad luck. However, the gained Curse appeared to be rather "Good Luck" rather than bad. In other words, the Demon is doing the opposite of the actions it tries to cause, most likely because they were originally heroes.
 * The "Four Heroes" concept is used again in Super Paper Mario, except this time with the four main characters of the Mario universe, and as the protagonists.
 * Though the Black Chest Demons are separate characters, the second says that the player fell for their trick a second time, probably meaning the second one knew of Mario's encounter with the first Black Chest Demon.
 * The Black Chest Demons are similar to a creature in The Legend of Zelda: A Link to the Past who doubles Link's magic meter, yet claims to halve his magic meter, and contradicts himself constantly after Link wakes him up.
 * Oddly enough, 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.
 * Oddly, only four of the Crystal Stars have been confirmed to be hidden by the Heroes.