The Master Sword is the legendary sword told of in Hyrulian tales of courage. It is the only sword created by the goddesses to be the complete bain of all evil, and as such is very hard to hard to obtain. Along with the Silver Arrows and Light Arrows, it is a reocurring element from the Legend of Zelda series that is rather hard to find and is required to defeat Ganon and his human form, Ganondorf.
The Legend fo Zelda: Ghost Tracker
After the young Link sets off to fulfill his great grandfathers uncompleted journey, he is alerted by Tyruce that the Master Sword is rumored to be hidden nearby, having relocated itself after it's journey completed. It appears later on after Skullpew Temple, when the Spirit of Power reveals that Ganon can only be defeated by the true bane of evil, the reawakened Master Sword, which could not fully kill Ganon last time because of hastiness in completion. After traveling to Stone Brewer Isle, the pirates learn that they must complete the 9 Goddess Challenges to unlock the 9 peices of the Triforce of Wisdom, which fully unlock the swords potential.
The Master Sword is confirmed to reappear in this game.