Majora was a ancient demon who was sealed away by the four Light Spirits in the Temple of Time. It would later be unsealed by a unwitting Link, leading to the boy having to go on a quest to stop it.
Majora's primary appearance is that of a purple cyclopean humanoid wearing robes laced with black and white. It's arms have multiple tendrils extending from them. Four puprle spikes covered in veins extend from it's back
Majora emerged from the earth along with the rest of his race, fleeing when Demise fell. He then associated with a non-human tribe, leading them to create a mask using a fraction of his power. However, he and the tribe were defeated upon their incursion into Hyrule. He was sealed away into the Temple of Time, with the Master Sword acting as the lock for his seal.
Majora slept until a squire Link discovered that he had lost his master's sword. Wandering into the Temple of Time, he drew the Master Sword which freed Majora from his prison. Using it's power, It teleported Link out of Hyrule Castle and lifted it into the skies. However, the young squire survived and discovered a Air Cannon. Using this, the young hero made his way to the ruins of Skyloft. Discovering a Loftwing, he then flew towards Hyrule Castle in order to slay the demon.
Upon entering Hyrule Castle, Link traversed the massive building on his way to Majora. Defeating each of the four guardians guarding the keys to the room Majora was in, he finally made his way to the ancient demon. Majora was no match for the Blade of Evil's Bane and fell to Link. As he died, Hyrule Castle fell to Earth, finally landing in it's correct place.
Majora is a powerful magic user, being able to easily lift Hyrule Castle into the skies. As well, he can channel his magic into powerful elemental attacks that can do a large amount of damage to foes. Majora's tendrils are able to channel this magic and can also be used as melee weapons in battle. Majora can resist most non-magical weapons and weaker spells without much effort. Majora can also create energy wings that grant him the power of flight.