Zelda Wiki:Sandbox

He then tells of how now that Skull Kid has the mask that the great catastrophe is going to befall Termina if it is not taken back from Skull Kid.

Though the game doesn't give much specific history on the origins of Majora's Mask, it does state that in ancient times, the mask was worshiped by an ancient tribe who used it in rituals involving the casting of hexes and curses, until they finally sealed it away from the world in the hopes that its evil power would not be unleashed ever again. Hundreds of years later is was sought out and found by the Happy Mask Salesman. Soon afterwards, the Skull Kid scared the Salesman unconscious and stole the mask from the salesman and began to access its power, awakening the consciousness of the mask.