Silver Arrow

"SECRET POWER IS SAID TO BE IN THE ARROW."

- Old Man

The Silver Arrow is a special upgrade to the bow and arrow used to defeat Ganon. They are the only weapon that is capable of defeating Ganon, who will recover from any wound if not pierced by a Silver Arrow. These arrows have appeared in both the first The Legend of Zelda and A Link to the Past. In later games of the series, the Silver Arrows are replaced with both the Light Arrows and the Master Sword, although the Light Arrows function differently in that they merely stun Ganon.

The Legend of Zelda
In The Legend of Zelda, the Silver Arrows are found in Death Mountain Labyrinth, the final dungeon. After hitting Ganon with his sword, Link must use the Silver Arrow to finish him off. Besides being able to defeat Ganon, these arrows make the double of damage that the regular arrows against every other monster.

A Link to the Past
In A Link to the Past, Silver Arrows are given to Link by the Fat Fairy inside the Pyramid of Power, once the way has been opened with a Super Bomb. These are more powerful versions of the typical arrows Link obtains, and are in most cases, the strongest weapon there is. When used as weapons in the field, they can kill most enemies in a single shot, including some bosses. Keeping in tradition with past games, the Silver Arrows are used to kill Ganon.

The Ancient Stone Tablets
In The Ancient Stone Tablets, the Silver Arrows appear from a giant stone slab that sealed away Ganon's Tower from the world, yet carry the same purpose and function as previous titles.

The Legend of Zelda comic
The Silver Arrow is also featured in two stories of the Legend of Zelda comic, and in both appearances, Princess Zelda is the one to wield it with her bow. In The Day of the Triforce, Princess Zelda decides to go attack and defeat Ganon once and for all since he is now powerless. Once reaches Death Mountain to challenge Ganon, she finds the powerless Ganon in his throne. Just as she is about to shoot him with the Silver Arrow, she hears a cry for help in another room and cannot help but to go and aid said person  instead.

In Choices, Zelda finds the Silver Arrow on a small pedestal after defeating a Darknut inside Death Mountain. She then recalls what Impa told her, about how, if Zelda's heart is true, then the Triforce of Wisdom will help her use the Silver Arrow to  destroy Ganon. When the princess finds Ganondorf, she wastes no time and quickly readies a Silver Arrow to shoot at Ganon, but before she has a chance to release it at the Prince of Darkness to  defeat him, Link comes crashing in, giving Ganon a chance to escape.

Trivia
The Silver Arrows' function as Ganon's only weakness is similar to more modern werewolf legends, in which a silver bullet is the only weapon that can kill a werewolf. This may be because silver was considered a holy element in many cultures.