Light Force
The Light Force is a source of limitless magical power featured in The Minish Cap that was gifted by the tiny Minish race to the race of Humans alongside the Picori Blade. It was the second of the two gifts given to the Hero of Men to combat the evil monsters overtaking the land when evil threatened Hyrule's stability. After the Picori Blade and the Light Force were used to successfully lock all the evils away within the confines of an enchanted chest, the Light Force eventually ended up in the female line of the Royal Family of Hyrule, where it has been known to give Hyrule's princess many magical and telepathic abilities. When the evil Vaati first traveled to Hyrule from the Minish Realm, he came seeking the Light Force and nearly succeeded in stealing it from its holder, until he was defeated and sealed away by Hyrule's hero, Link. Many theories have sprang up around the Light Force in the years following the release of The Minish Cap, several of which have linked it to the Triforce or more specifically the Triforce of Wisdom, of which Hyrule's princess is also known to be the keeper.
History
The Legend of the Hero of Men
Long ago, the land of Hyrule 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, a young man, the Hero of Men, stepped forth to combat the creatures and save Hyrule from destruction. It was then that the tiny Minish race of beings 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 (in the form of a single golden triangle similar in appearance to a piece of the Triforce). Using these two tools in unison, the Hero of Men was able to drive back the evil monsters overtaking Hyrule and magically sealed them all away within the confines of the mythical Bound Chest, sealing the chest with the Picori Blade itself. After the conflict's end, it is unclear what became of the Hero of Men or what he did with the Light Force. However, it is clear that at some point after the conflict, the Light Force was magically sealed within the body of Hyrule's princess and was passed down to her successors throughout the generations as it ran through their bloodline. This secret of the Light Force's true fate was unknown to even the best of Hyrulean historians and 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.
The Minish Cap
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 become a powerful new being free of the constraints of the tiny size of his own race, the Minish. Vaati eventually discovered the story of the Light Force and felt that it could grant him the power he sought, and he grew to desire to seek it out and use it to escape the life he lived under his master, the renowned Minish sage, Ezlo. One day, Vaati discovered a magical cap 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 Hylian sorcerer and further fueling his evil desires for the domination of the world. Ezlo discovered his apprentice's transformation and learned of his plan to seek out and claim the Light Force to become all-powerful. Ezlo tried to stop him but was transformed by Vaati into a similar cap with the head of a bird on top. As Vaati escaped through the Minish Door that opens only once a century to Link the Minish Realm and Hyrule together, he learned of the annual Picori Festival and its sword-fighting tournament, one of the prizes for which is the honor of being given access to the Bound Chest spoken of in the legends of the Hero of Men and touching the Picori Blade. Vaati, under the impression that the chest held the Light Force itself, entered the tournament and used the Minish Cap's great magical power to win the tournament in record time. When he was granted access to the chest however, he cast back the guards with his magic and blasted open the Bound Chest, releasing all the monsters held within into to 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 hinderance 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. Had he been paying more attention to Zelda, he would have noticed the "force of the golden light" eminating from her the moment he blasted open the Bound Chest, but the evil sorcerer had failed to see it. 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 attempmt 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 Minish 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. Link defeated Vaati with the Four Sword and sealed him within the Four Sword's blade, causing the evil wind sorcerer to leave only the Minish Cap behind. With Vaati's defeat, Zelda was restored to life and Ezlo was restored to his true form. Ezlo placed the Minish Cap atop Zelda's head and told her to combine its power with the Light Force that still rested within her to undo all of Vaati's evil. This succeeded, but the Minish Cap fell apart. It is unclear whether or not the portion of the Light Force stolen by Vaati was restored to Zelda upon his defeat, but Ezlo makes clear before his return to the Minish Realm that Zelda still possesses the Light Force within her.