Light Force

The Light Force is a powerful form of the life force within all living beings, distinct from the Triforce, that appears in. Given by the Minish race to the people of Hyrule alongside the Picori Blade at an early point of the Force Era of Hyrule's long history, it was essential to the efforts of the legendary Hero of Men during the so-called War of the Bound Chest, enabling him to drive back and lock away an army of evil creatures threatening the land. It was later, like many other sacred objects, passed down within the Royal Family of Hyrule, albeit as an intangible essence within the bloodline of Princess Zelda.

Skyward Sword
When the Master Sword was blessed by Zelda, the mortal incarnation of Hylia, it came to possess the Light Force, the source of all life in the world.

The Legend of the Hero of Men
Long ago, at an unspecified point after the Era of Prosperity, in which the Kingdom of Hyrule was established, and during the Force Era, the world was on the verge of being swallowed by shadow, plagued by evil monsters and the atrocities they wrought upon it. As doom seemed near at hand, the Hero of Men, stepped forth to combat the evil creatures and save the Kingdom of Hyrule from the brink of destruction. It was then that the tiny Minish descended from the skies, bringing the Hero of Men a magical sword called the Picori Blade and a source of great magical power called the Light Force. Using these sacred gifts, the Hero of Men was able to drive back the monsters and magically sealed them all away within the Bound Chest. The Light Force was then sealed within the body of Hyrule's princess and was passed down to her successors through the generations. This secret of the Light Force's true fate was only documented in one place: the stained-glass windows of a secret chamber within the Elemental Sanctuary that connects Hyrule to the Minish Realm.

Many long years following the conflict involving the Hero of Men, a young Minish apprentice by the name of Vaati became enchanted by the evil that could come from the heart of man and began to seek a means to gain power. He learned of the legends of the Light Force and hoped to find it within the Bound Chest.

One day, Vaati discovered the Mage's Cap that his master, Ezlo, had created as a gift for the Humans that would grant the wishes of its wearer. Vaati took the cap and put it on without permission, transforming himself into a powerful sorcerer. Ezlo tried to stop him but was transformed by Vaati into a similar cap with the head of a bird on its top.

As Vaati escaped through the Minish Door to Hyrule, he learned of the annual Picori Festival and its sword-fighting tournament. The winner of this tournament is given the honor of touching the Picori Blade that sealed the Bound Chest. Vaati, under the impression that the chest held the Light Force itself, entered the tournament and used the great magical power of the Mage's Cap to win in record time. When he was granted access to the chest, he cast back the guards with his magic and blasted open the Bound Chest, releasing all the monsters held within back into the world, and shattering the Picori Blade. Seeing Princess Zelda nearby, Vaati knew she had great power that ran through her bloodline (though he did not know its true nature) and felt she would only be a hindrance to him if he allowed her to be free. So the evil sorcerer used his magic to turn Zelda to stone and inspected the chest, only to find his theory regarding the Light Force's location to be wrong. Deciding he would be patient and still find the Light Force wherever it lay in Hyrule, Vaati dissipated into thin air, fleeing the scene.

As Princess Zelda's childhood friend, Link, journeyed around Hyrule gathering the mystical Four Elements in an attempt to reforge and repower the Picori Blade and break Vaati's curse on Zelda, Vaati concocted a scheme to use Hyrule's military force to search the land for the Light Force by impersonating the ruling King of Hyrule, Daltus. As such, Vaati infiltrated Hyrule Castle and turned the King to stone, hiding him in one of the dungeon cells of the castle and using the Mage's Cap to take on his appearance. He then ordered the surprised guards to search the entire land for the Light Force and ordered cruel punishments to be inflicted upon those who dissented. As Link approached the completion of his goal of acquiring all of the Four Elements, Vaati eventually discovered the existence of a secret chamber in the Elemental Sanctuary in Hyrule Castle Garden that would only open and yield access to the secret of the Light Force to one holding the sacred sword. With this knowledge, the evil sorcerer decided to wait for Link to recharge the sword and use him to gain access to this special chamber. This plan succeeded, and Vaati discovered the location of the Light Force from the windows. Armed with this knowledge, Vaati transformed Hyrule Castle into Dark Hyrule Castle and took the petrified Zelda to the roof, intending to extract the Light Force from her, a process she would not likely survive. Link and Ezlo meanwhile fought their way through the structure and reached the roof just in time to stop Vaati from completely extracting the Light Force. Believing he had enough of it to defeat Link, Vaati fought Link in three deadly forms, the last of which he finally took on his classical appearance of a one-eyed wind demon.

The battle ended, with Vaati defeated and the Mage's Cap remaining intact. With Vaati's defeat, Zelda was restored to life and Ezlo was restored to his true form. Ezlo placed the Mage's Cap atop Zelda's head and told her to combine its power with the remaining Light Force that still rested within her to undo all of Vaati's evil. This succeeded, but the Mage's Cap fell apart. The Minish Door was about to close, so Ezlo bade farewell and returned to the Minish Realm.

Trivia

 * If Link takes too long in the final room before reaching Vaati in Dark Hyrule Castle, Vaati's ceremony will complete. He will then drain the Light Force from Zelda, resulting in her death.

Nomenclature
The Japanese name for the Light Force, Life Force, and Force Gems is simply "Force."