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. Only two swords of the series have this ability: the Master Sword, which is called the "Blade of Evil's Bane" for this very reason, and the Four Sword.

History
The power to repel evil beings with the creation of the Master Sword, as told in Skyward Sword. Though the mythical blade The Goddess Sword possessed great power, Link finds himself in need of a Gate of Time, and his sword will be unable to activate it until he forges it in the flames of three goddesses. Farore and Nayru give Link standard improvements to his blade, but it is Din's flame, the red flame of the Goddess of Power, that imbues his sword with the mystical force that repels evil. It is here when the mysterious Goddess Sword is transformed into the legendary Master Sword. Upon returning his blade to the past and discovering the origins of the Goddess Hylia and Zelda, the symbols of the Triforce show that Link is a hero worthy of wielding the true golden power, and as such, that same power is infused in the Master Sword, turning it into the True Master Sword, solidifying it as the blade of evil's bane that only Link may wield.

This power would be continually awakened within the Master Sword when Link, who shares the spirit of the hero, draws the blade. However, it is not until The Wind Waker that this power is lost from the blade. The prayers of the Earth and Wind Gods, which continue to keep the Goddess' power within the Master Sword, are silenced by Ganon. The Master Sword is used to seal Ganon's power, and though drawing the blade grants him his full power, it is also the only thing capable of defeating him, regardless of his strength. Knowing this, Ganon exploits a loophole which causes the Master Sword to be drawn by the hero, but without the power to repel evil, it cannot defeat him, thus granting Ganon his full powers without the threat of the Master Sword's revival. It is only by establishing news sages that the Master Sword is allowed to realize its full potential as the True Master Sword once again.

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.

In Twilight Princess, Zant casts a spell on Link, turning him into Wolf Link without the effects of Twilight. Upon seeking the Master Sword, the evil that curses Link is driven away from him, and he is allowed to draw the sword for his own. Midna then remarks that the item Zant uses to curse Link is different compared to the usual shadow magic of the Twili. Midna decides to keep it, and by using the alternating effects of the evil magic and the Master Sword's power to repel it, Link gains freedom over his transformations.

Along with revealing the origins of the power, Skyward Sword also reveals 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.

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.

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.
 * In Four Swords Adventures, Force Gems are the only things keeping the Four Sword's power to repel evil intact, thanks to Vaati's evil magic.
 * 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.