One of many butterfingers from the Zelda games. The Legend of Zelda: Majora's Mask states that Majora's Mask was created by an ancient tribe and was used in rituals, but nothing is known of Majora him/herself. He/she/it was probably the original owner of Majora's Mask or had a large part in its creation or something. The spirit of Majora is supposedly living inside Majora's Mask, and possesses whoever wears it. It is also safe to guess that Majora was pretty evil.
In the game, Majora took possession of the Skull Kid and used him to cause Termina's moon to fall out of its orbit and come crashing down on its major city Clock Town on the day of the Carnival of Time. Initially Link was unable to stop Majora from fulfilling this evil desire, but with the help of the Ocarina of Time and the four giants that he summoned from the four regions surrounding Termina, the moon was prevented from touching ground. Majora then discarded the Skull Kid and beamed himself to the moon, where Link confronted him in his three forms: Majora's Mask, Majora's Incarnation, and Majora's Wrath. After defeating Majora's final form, the moon disappeared from sight, and the mask that was now cleansed of its evil was returned to the Happy Mask Salesman.