Silver Arrow

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

- Old Man

The Silver Arrow is a special upgrade to the Bow & Arrow used to defeat Ganon. They are the only way to finally kill Ganon, who will recover from any wound if not pierced by a silver Arrow. These arrows appeared in both the first Legend of Zelda and A Link to the Past. In later games of the series, these arrows were replaced with both the Light Arrows and the Master Sword.

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 uses the Silver Arrow to finish him off. Besides being able to finish off Ganon, these arrows work exactly the same as 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 arguably some of the most potent weapons available. When used as weapons in the field, they can kill most enemies in a single shot, including some bosses. Once again, they 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.

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.