Guardian Deity

In Majora's Mask, the Four Giants are the guardian gods of Termina that rest in the four cardinal directions of the land of Termina: Woodfall, Snowhead, Great Bay and Ikana Canyon. Every year during the Carnival of Time, the people of Termina ask the Four Giants for a rich harvest in the year to come, wearing masks resembling these deities to honor them.

History
Before the events of Majora's Mask take place, long ago the people of Termina lived in a single world, united and blessed by the four giants living among them. It was during this time that the giants befriended a small imp. However, there came a time when the Four Giants decided to guard and protect the people while dormant. With the world of Termina now split into four parts by the order of the Four Giants, each of the giants took one hundred steps towards their respective regions, telling their people that if trouble should befall them, they should call them in a loud voice declaring their predicament. This way, their pleads would carry to them, and the Four Giants would come to their aid.

However, the small imp, who had been friends with the giants even before they created the four worlds, became greatly afflicted with their decision and was woefully stricken by their departure. The imp misinterpreted the giants' actions, and, feeling neglected and lonely, he began to spread his anger and frustration throughout all of Termina. Devastated with misery due to the imp's malicious tricks, the people pleaded to the giants for their aid. Angered, the Four Giants, who were still the protectors of the people, ordered the imp to leave the four worlds and return to the heavens; otherwise, the imp would be destroyed by the might of the giants. Frightened and surprised by his friends' reaction, the imp left the lands for the heavens, and peace and tranquility returned to Termina.

For reasons unknown, the imp, known as the Skull Kid, managed to fall from heaven and return to the world of Termina. Eventually, the Skull Kid obtained Majora's Mask from the Happy Mask Salesman. Overtaken by its tremendous power, the mask began to take complete control of the Skull Kid, and using him as a puppet, he pulled the moon from its orbit and destined it to crash into Clock Town in order to destroy all of Termina.

After stumbling into Termina and having befriended one of the Skull Kid's fairies, Link and Tatl set off in an adventure in order to stop the small imp. When the two heroes encounter the imp atop the Clock Tower for the first time, Tael, who was still with the Skull Kid, informs the young hero and his fairy companion about the Four Giants who are trapped in each of the temples that are located in the four compass directions. One by one, Link and Tatl free the giants and lift the seal that was put into effect by Majora's Mask and the little imp. The giants in return teach Link the Oath to Order, which is to be used to summon the giants on the eve of the Carnival of Time.

With the giants free from their seal, Link and Tatl head to the top of the Clock Tower in order to call upon the four gods using the melody as taught by them. Together, the giants use their tremendous power to stop the moon from obliterating all of Termina. However, in a turn of events, Majora's Mask tosses aside the body of the Skull Kid and enters the moon itself to continue with its path of destruction. The Four Giants struggle to keep the now more powerful menace from descending into the land while Link and Tatl venture to the surface of the moon. After defeating Majora, the moon vanishes in a beam of light, never to be seen again. Soon after, the Giants reassure the Skull Kid of their friendship with him, and having done so, the four gods depart again to their respective regions.

Trivia

 * In the event that Link fails to awaken all four giants by the time the moon falls, the giant(s) who were awakened will arrive at Clock Town to attempt (but ultimately fail) to stop the moon.
 * The Four Giants appear in the background of the Termina Great Bay stage of Super Smash Bros. Melee, and stop the the moon from falling like they do in Majora's Mask. They also have their own trophy that can be randomly obtained through either the lottery or one of the player modes; however, the trophy misquotes Tael, and uses the word "Valley" instead of "Canyon".
 * Head Statues that appear in Phantom Hourglass and Spirit Tracks resemble heads of the Four Giants.