Power to Repel Evil

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.

Use
The power to repel evil is mostly used to defeat the game's primary antagonist, like 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. 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. You can only destroy them of you collected enough Force Gems to give the Four Sword the power to repel evil.

Trivia

 * In 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.
 * 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.