Power to Repel Evil

"You cannot defeat me with a blade that does not sparkle with the power to repel evil! What you hold is useless."

- Ganondorf, The Wind Waker

In The Legend of Zelda series, the Power to Repel Evil is the ability to banish pure evil. The two main swords of the series (the Master Sword, which is called the "Blade of Evil's Bane" for this very reason, and the Four Sword) have this ability.

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The power to repel evil beings was first displayed during the creation of the Master Sword, which occured during the events of Skyward Sword. Originally known as the Goddess Sword, the weapon possessed a minor version of this power, and was used by the goddess Hylia in her fight with the Demon King, Demise. However, when Link later finds himself in need of a Gate of Time, his sword is unable to activate it until he forges it in the three Sacred Flames. The flames of Farore and Nayru improve the physical strength of the blade and its dowsing capabilities, respectively, but it is Din's flame that imbues his sword with "a sacred white light that demons revile," a form of the power to repel evil, and at the same time transforms it into the legendary Master Sword.

Shortly thereafter, Link uses his new blade to activate the Gate of Time and travels to the past. There, after explaining her history and plan, Zelda blesses the sword, turning it into the True Master Sword. This transformation augments the power granted to the blade by Din's Flame, causing it to actually shine with that power.

It is also revealed that the blade wields the power to absorb evil. Upon the defeat of Demise, the blade's mystical qualities not only slay him, but he is completely dissolved. His residual consciousness is absorbed into the blade, where the power keeps him trapped long enough for him to decay within the sword.

In Twilight Princess, Zant casts a spell on Link, keeping him trapped in his wolf form. Princess Zelda is quick to realize that only the Master Sword can break through Zant's curse and return Link to normal, and instructs Link to find it. Once he does so, the power to repel evil pours out of the blade, driving out the curse and condensing it into a Shadow Crystal. Midna remarks that the item is different compared to the usual shadow magic of the Twili, before deciding to keep it. By using the alternating effects of the evil magic and the Master Sword's power to repel it, Link gains the ability to transform, and warp, at will.

In The Wind Waker it is revealed that at some unknown point, the power to repel evil became dependent on the prayers of the Earth and Wind sages, which continue to keep the Goddess' power within the Master Sword. These prayers are silenced when these sages are killed by Ganon, forcing Link to seek out their successors to restore the power. After awakening the new Wind Sage, the power to repel evil is restored.

In and, the Four Sword is shown to repel evil influences, but it is not until Four Swords Adventures that this power is called such. In this same game, it is revealed that the sword initially lost this power due to containing Vaati and his evil magic for so long; it is also revealed that this power comes from Force Gems. In each level, the Links must gather 2000 Force Gems to restore the power to repel evil. This power allows the Links to kill Vaati and the various enemies throughout the game, as well as to defeat Ganon.

Use
The power to repel evil is most often used to defeat the game's primary antagonist, Ganondorf. However, it has been shown to have other uses. For instance, in A Link to the Past, Link uses this power to reflect Agahnim's fireball spell. In The Wind Waker, Link uses the Master Sword, infused with the power to repel evil, to destroy the dark barrier surrounding Hyrule Castle, thus giving him free roam of the path to Ganon's Tower. The Power to Repel Evil is infused in the Master Sword with the help of the Sages of the Master Sword. Similar to the Light Arrows, the power to repel evil is the only thing that can hurt Ganondorf.

In Four Swords Adventures, at the end of every level there are dark magic barriers blocking entrance to the next level. The Links can only destroy them if they collect at least 2000 Force Gems to give the Four Sword the power to repel evil.

Other Sources
Although the Master and Four Swords are the clearest, most powerful, and most versatile examples of the power to repel evil, this power seems to be displayed by other aspects of the Zelda universe as well, such as:
 * The Kokiri Forest, and in particular the Great Deku Tree and his child who preside over it, are shown to keep out evil creatures and influences. The forest had virtually no monsters originally and served as a safe home for the Kokiri prior to the death of the Great Deku Tree; some time after his death, it became filled with monsters; and upon the growth of the Deku Tree Sprout, they disappeared once more.
 * The Six Sages appear to have a limited form of this power, being able to keep Ganondorf's influence out of the Chamber of Sages despite his takeover of the Sacred Realm, and later being able to bind his power and seal him away. The Maidens descended from them also seem to have a limited form of this power, being able break the barrier on Ganon's Tower (in the Downfall Timeline) and seal Ganon within the Four Sword (in the Child Timeline).
 * The Sacred Shield and its upgrades in SS undefined are shown to be able to repel certain curses and dark influences, such as Cursed Bokoblins.
 * Light Arrows and Silver Arrows are able to actually pierce evil itself and burn the essences of those they pierce.
 * The Mirror Shield may have the power to repel evil, as it is shown to be able to repel attacks by Twinrova (in its incarnation) and nullify certain attacks by Ganondorf and others; although, this may be due to its reflective properties as a mirror, rather than the actual power to repel evil.
 * The Phantom and Lokomo swords are both imbued with sacred energy and are the only weapons (outside of the Bow of Light and weapons enhanced by Tears of Light) than can kill and stun Phantoms, respectively.

Trivia

 * In The Wind Waker, the Master Sword's power to repel evil was only infused into the blade when the Earth Sage and the Wind Sage pray to the gods for its continuous power by playing the Earth God's Lyric and Wind God's Aria, respectively.
 * It is unknown whether the Triforce's power, which plays an integral role in creating the True Master Sword in Skyward Sword, holds any bearing on the sword's power to repel evil. As is constantly shown, evil beings have no difficulties in using the Triforce, and the Triforce's connection to the Gods is shown to be completely impartial to those who use and wish upon it.