White Sword of the Sky

The Goddess Sword is an item featured in Skyward Sword. It is inhabited by a spirit known as Fi. Eventually, the Goddess Sword becomes the Master Sword.

History
The Goddess Sword was the sword once wielded by the Goddess Hylia. During her war against Demise, Hylia created Fi, and enshrined the sword Inside the Statue of the Goddess before raising it into The Sky as part of Skyloft.

Preceding the Wing Ceremony, Fi appears to Link in his dreams, foretelling him of his destiny. After Link and Zelda are attacked by a black tornado, Link wakes up in his room, where he hears Fi speaking to him. Fi guides him to the Inside of the Statue of the Goddess and further explains his fate as a hero and encourages him to draw the Goddess Sword from its pedestal. Fi gives Link the Emerald Tablet and tells the young hero that he must go beneath the clouds in order to find Zelda and fulfill his destiny and that the Emerald Tablet will guide the way.

Later, Link must temper the Goddess Sword using sacred flames, first being tempered by Farore's Flame, resulting in the Goddess Longsword. Then it must be tempered with Nayru's Flame, creating the Goddess White Sword. After it has been tempered by Din's Flame, it become the Master Sword. Once the Master Sword has been blessed by The Goddess, it becomes the True Master Sword.

Powers and Abilities
The Goddess Sword houses a spirit, Fi, who guides Link in many ways, such as giving him helpful advice or actually pointing him in the direction he needs to go. Moreover, the sword has a "treasure-seeking ability" called Dowsing. It can sense important people and items and inform Link about them. When Fi has something to say, the sword begins to glow.

If Link holds the sword up to the sky, it will be charged up by "energy from the heavens" and be able to release a Sword Beam once it's swung. This beam can activate Goddess Cubes, sending them into the Sky.

Enhancements
The Goddess Sword is enhanced multiple times over the course of Skyward Sword. The first three alterations are induced by tempering it with the three Sacred Flames. First, Farore's Flame converts it to the Goddess Longsword, which has doubled attacking power and greater reach. Then, Nayru's Flame converts it to the Goddess White Sword, which adds the ability to dowse for Rupees, Treasures, Gratitude Crystals, and Goddess Cubes. Finally, Din's Flame converts it to the Master Sword, with three times the original Goddess Sword's attacking power and a faster charge time for Skyward Strikes. However, this Master Sword resembles the Master Sword as it is first seen in The Wind Waker, in its weakened state.

The final change to the sword takes place in the past, before Zelda enters into a deep sleep to keep the seal placed upon Demise intact. Her blessing converts it to the True Master Sword, with the familiar Master Sword appearance seen throughout the series. With this final transformation the sword becomes able to harm Demise.

Theory

 * Hylia's primary antagonist, the Demon King Demise, possesses a sword that seems strikingly similar to the Master Sword. It remains unexplained whether the blade wielded by Her Grace and her chosen hero was specifically created to counteract Demise's weapon or vice versa. This is made further unclear by the fact that Fi and Ghirahim are similar humanoid beings accompanying each sword.