Black Chest Demon

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

- Black Chest Demon

Mario encountered four Black Chest Demons 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 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 who was once part of the demonic army 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 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.

Airplane Mode

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

Paper Mode

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

Tube Mode

 * Chapter Obtained: 4
 * Power obtained in: Twilight Town
 * To use power: Press R and rotate control stick. Press B 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 Y and press Y 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.