Light Force

The Light Force is a source of limitless magical power featured in The Minish Cap that was given by the Minish race to the race of Humans alongside the Picori Blade. 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 Hyrule long in the past. It also played a central role in the story of The Minish Cap, where the villainous Vaati sought to obtain it and become god-like in strength. Combining the Minish Cap's power with that of the Light Force, Princess Zelda was able to restore Hyrule to its former state of peace and undo all of Vaati's evil deeds at the end of the story.

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, the Hero of Men, stepped forth to combat the creatures and save Hyrule from 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, the Hero of Men was able to drive back the evil monsters and magically sealed them all away within the Bound Chest.

The Light Force was magically 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 World.

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 gain power. He learned of the legends of the Light Force and hoped to find it within the Bound Chest.

One day, Vaati discovered a magical cap 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 Hylian 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 Minish Cap's great magical power 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 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 began infiltrating 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.

The battle ended, with Vaati defeated and the Minish Cap remaining intact. 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 remaining Light Force that still rested within her to undo all of Vaati's evil. This succeeded, but the Minish Cap fell apart. The Minish Door was about to close, so Ezlo bidded farewell and returned to the Minish Realm.

Description
The Light Force is depicted in The Minish Cap to be a "force" that is embodied in Hyrule's Princess Zelda, and it is seen eminating from her for a brief moment when Vaati blasts open the Bound Chest believing it to actually be inside the chest itself. In a physical descritpion of the relic however, the Light Force is depicted as being a single golden triangle in the fashion of the three pieces of the Triforce. While the triangle form of the Light Force is not physically seen in-game (only as a golden light eminating from Zelda both when Vaati blasts open the chest and when she uses the Minish Cap to heal Hyrule), it can be seen on the opening file menu for a file on which the game has been completed, and it is depicted as it is in the stained-glass windows, as a single golden triangle beside the Picori Blade/Four Sword. As far as how powerful the Light Force really is, this is only hinted at by Vaati and Ezlo. Vaati states that the Light Force would make him like a "god", and Ezlo states that its power is nigh limitless, though just how powerful this would be is debatable, as the most powerful relic in the entire series is the Triforce.

The Light Force as the Triforce of Wisdom
Some have come to believe that the Light Force is merely a different incarnation of the Triforce of Wisdom, of which Princess Zelda is also known to be keeper and guardian. This is largely due to the appearance of the Light Force depicted in the stained-glass windows as well as within the completed game menu for The Minish Cap, which depicts it to be a single golden triangle that looks identical to a piece of the Triforce. This theory however, like almost all others, has some flaws to it as well. First, the game developers have hinted themselves in interviews that the Light Force is independent of the Triforce and is its own separate entity. There is also the backstory of the Light Force itself: it is stated that the Light Force was given as a gift by the Minish to the Hero of Men to help him in his fight against evil. This is in stark contrast to the story of the Triforce of Wisdom, which states that it was created by the Goddess of Wisdom, Nayru, and fused with the two other Triforce pieces to form the one true Triforce. The two stories do not appear to match one another, making it less plausible the two are one and the same. This is not to say however that the Light Force may perhaps have been the Minish race's attempt to create something akin to the Triforce itself and may have been their attempt to in fact create a relic that would be comparable to it in power and strength.

Tetraforce
There are also other fans of the series that have theorized that the Light Force may in fact be a missing fragment of the Triforce that was broken apart from the other three, the Light Force being the piece that would have occupied the central triangular hole. This has been due to the vision of four triangles eminating from Zelda when she puts on the Minish Cap and uses it to cleanse Hyrule of evil. Some believe this marked the creation of the Triforce and the separating of the Light Force from the three other pieces. However, this theory also has multiple holes that make it inconsistent with plausibility. First, the Triforce itself appears on banners and even on Zelda's clothing within the game, meaning the kingdom was already well aware of the relic's existence, and there are only three pieces depicted in all these pictures of it throughout the game. Secondly, the Triforce is stated to have been created by the three Golden Goddesses, and each only made a single golden triangle that combined together to make the Triforce at Hyrule's creation. There is no mention of any other piece being created at that time, and even all the scenes throughout the series depicting the creation of Hyrule and the Sacred Realm even show there was no central piece ever present. These inconsistencies, along with others, also make this theory less plausible.

Force Gems
Another theory states that the Light Force could have been a giant Force Gem and when Vaati was defeated in Minish Cap, the Light Force may have shattered into Force Gems as they are used to power up the Four Sword, also in Four Swords Adventures, as you use the Down Thrust technique on Vaati, he loses Force Gems each time. Yet another theory states that the Four Sword also possessed some of the Light Force, and when Vaati's magic robbed the Four Sword of its power to vanquish evil, its magical energy shattered into Force Gems as well.

The Essence of Hylia
In Skyward Sword, which precedes all other Zelda games in the timeline, Zelda is the Goddess Hylia in human form. There is a theory that states that the Light Force is, in actuality, the holy essence of this Goddess that Zelda passed down through her bloodline. However, the Minish are not mentioned in Skyward Sword, and it was Hylia herself who sacrificed her divine form to become Zelda.

Trivia

 * The Light Force in a way mirrors the Fused Shadow of the Twili that was an immensely powerful relic made by one of the various races of Hyrule that was said to rival the Triforce in terms of power. But it is unlikely that, like the Fused Shadow, the Light Force would be more powerful than the Triforce, as the Triforce was made by the powers of the goddesses themselves, whereas the Light Force was a powerful relic made by the magic of mortal beings.