Light Force

The Light Force is a powerful energy source that dwells inside Princess Zelda in The Legend of Zelda: The Minish Cap. Vaati drains most of it, thus turning himself into a powerful being.

Origins
According to legend, when evil threatened Hyrule's existence during the War of the Bound Chest, the world was on the verge of being swallowed by shadow. Then, when all hope seemed lost against the Army of Evil, the tiny Minish race appeared from the sky, bringing the Hero of Men a sword known as the Picori Blade and a golden light known as the Light Force. The Light Force, in appearance, looks not too unlike one of the three component golden triangles that make up the all-powerful relic known as the Triforce. In this case, it appears as a single golden triangle and acts as a source of limitless magical power, although just how powerful the Light Force is compared to the might of the Triforce is presently unknown.

The evil Minish Vaati claims it has the strength to turn him into a god, but this was never substantiated since he was defeated by Link on more than one occasion. It is presumed that the Light Force originates from the Minish race itself since they gifted it to the human race, but just how they created such an energy source is still unknown. It would seem that the more powerful Minish, the "Sages" (although Ezlo was the only one who seemed to exist in the events of The Minish Cap), could create incredible items such as Ezlo's prized creation, the Minish Cap, which grants the wishes of its wearer and has similar qualities to the Triforce itself. Assuming there were more Sages present at the time of the war, it is possible that they could create such a relic.

Presuming that the Light Force was indeed a creation of the Minish, it would then mirror the Fused Shadow of the Twili, which was also a source of powerful magic created by another of Hyrule's races. However, as was seen in The Legend of Zelda: Twilight Princess, even the Fused Shadow was no match for even a third of the Triforce, so it may be presumed that the Light Force is by the same token not nearly as powerful as the Triforce (although it remains an immensely powerful artifact) or even a part of it since the Triforce is the power of the Goddesses themselves rather than a mere creation of one of Hyrule's races.

The Hero of Men combined the power of the Picori Blade with the power of the Light Force to bind the entire Army of Evil within the Bound Chest spoken of in Hylian lore. After the war had been won, the Hero of Men sealed the Bound Chest with the Picori Blade itself, keeping all evil locked away within the depths of the chest. The Light Force however disappeared, and not even Hyrule's best historians could be sure of what happened to it. The secret of the Light Force's fate was only depicted in one place: a series of stained glass windows within the Elemental Sanctuary that links the Minish World to Hyrule. The last of these windows revealed that the Light Force, upon completing its role in assisting the Hero of Men in the War of the Bound Chest, was secretly sealed within Hyrule's Princess Zelda, and was subsequently magically passed down to her heirs in succession.

The Legend of Zelda: The Minish Cap
When the evil Vaati comes to Hyrule to search for the Light Force centuries later, he believes the Light Force had been hidden within the chest itself, only to find his theory to be incorrect when he shatteres the Picori Blade and openes the chest, releasing the Army of Evil that had long been imprisoned within. Shortly thereafter, he turns the current Princess Zelda to stone believing she would be an obstacle in his search for the Light Force. Had he been watching more closely, he would have seen the "force of the golden light" emanating from her the moment he blasted open the Bound Chest. The petrified princess was then placed in Hyrule Castle's Throne Room until the Picori Blade could be restored and used to break Vaati's curse.

Eventually, Vaati incapacitates the King of Hyrule and impersonates him in order to use the castle soldiers to search the land for the Light Force. After weeks of no success, he learns of the Elemental Sanctuary's existence and learned that when the Sacred Sword is infused with the power of the Elements, the way to the Light Force will open. Knowing Link was searching for the elements, he decided to wait for Link to infuse the blade with all four elements and use him to discover the Light Force's true resting place. This plan succeedes when Link returned from the Palace of Winds with the Wind Element and infused its power into the blade to restore the Four Sword. Just then, the door to the chamber containing the stained glass windows opens, allowing Link and Ezlo access to the windows.

There, they learn the truth about Princess Zelda and the Light Force, but a high cold laugh rings throughout the chamber as Vaati appears and incapacitates the pair, finally having learned that the Light Force had been inside Princess Zelda all along. Using the power of his magical cap stolen from Ezlo, Vaati petrifies all of Hyrule Castle's inhabitants and transforms it into Dark Hyrule Castle, taking the petrified Zelda to the castle roof.

While Link fights his way to the top of the castle, Vaati begins a process to extract the Light Force from Princess Zelda, a process that would have left her dead if completed. Link makes it just in time to stop Vaati from fully obtaining the Light Force, but the evil sorcerer believes he had drained enough of it to combat Link and win. However, Vaati was wrong, for the power of the Four Sword combined with Link's skills defeats him. It is unknown whether the greater portion of the Light Force Vaati obtained from Zelda is returned to her at this point. However, Ezlo makes it clear that she still possesses at least some of the Light Force. Using the Minish Cap and combining its power with the power of the Light Force, Zelda purges all the evil created by Vaati.

Theory
Some believe that the Minish created the Light Force to fit in the center of the Triforce. Together the Triforce and Light Force would grant unlimited power. Another theory is that when Vaati was defeated the Light Force was shattered into Force Gems as they are used to power up the Four Sword.