From the Super Mario Wiki
The Crystal Stars are the seven stars Mario and his partners collect in Paper Mario: The Thousand-Year Door, and are the game's objectives. Upon collecting one, it will teach Mario a Special Move. When all seven are collected and shown at the pedestal in Rogueport Sewers, they will open The Thousand-Year Door to the Palace of Shadow.
The stars were created by the Shadow Queen to hold the essence of the heavens, and were placed in several locations to further expand her power. One of the castles built to hold the stars is Hooktail Castle. However, four heroes collected them and used their power against the Shadow Queen, but the heroes were cursed before the fight finished, and they couldn't let go of the Crystal Stars or else they will be trapped in a black chest. Knowing this, the heroes gave the Crystal Stars to others to protect.
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