Black Chest Demon

There are four Black Chest Demons in the video game Paper Mario: The Thousand-Year Door. To open their chest, Mario would need to find a special black key. Once he does, the Demon will emerge and will curse Mario. While the Demon assumes that the curse is bad, it actully benefits Mario in the longrun by giving him special abilities. There is actually a story behind the four demons that Grifty in the sewers tells you.

The story behind the four Black Chest Demons
Over one thousand years ago, there were four heroes who planed to stop the evil Shadow Queen. The four included a strong Toad, a Koopa Troopa, an intelligent Goomba, and a Boo. During the fight against the Queen, they were able to seal her away. Unfortunately, the demon cursed the four of them right before they sealed her way. This curse was that if they would ever let go of the Crystal Stars, a Black chest would come out of no where and lock them up for eternity. Having knowlage that the seal would not last forever, the four of them wanted these stars to people who needed them.

They also created the map at the start of the game. They put the map inside a chest that could only be opened by someone with a pure heart (aka Princess Peach).