White Sword of the Sky

Crafted by the Goddess Hylia, the Goddess Sword is a legendary blade featured in SS undefined, which is later tempered into the Master Sword by Link. It is inhabited by a spirit known as Fi, who aids Link throughout his journey so that he may fulfill his role as Hylia's chosen hero.

History
The goddess Hylia created the Goddess Sword and Fi, an intelligent humanoid spirit who resides in the sword, and gave it the purpose of assisting Hylia's chosen hero. She then 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, further explaining his fate as the chosen hero and encourages him to draw the Goddess Sword from its pedestal so that Fi can formally acknowledge him as her Master. 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, with the tablet illuminating the path ahead.

After Link receives the Goddess's Harp from Zelda, the young hero must temper the Goddess Sword using the three Sacred Flames in order for it to hold the necessary power to open the Gate of Time at the Sealed Grounds. Each time Link bathes the Goddess Sword in one of the flames, the sword is empowered and is physically altered, giving it new powers. 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 four 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, 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: the cross-guard's position extends outward, and the gem and the Triforce symbol are restored. With this final transformation, the sword becomes able to harm and defeat Demise. Additionally, the time it takes to charge a Skyward Strike is reduced. In Hero Mode, the Skyward Strike is already at its maximum power and charging speed from the previous playthrough, so upon acquiring the True Master Sword its charge time is upgraded again to become near-instantaneous.

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. When Fi has something to say, the sword begins to glow, and Link can speak to Fi at any time by pressing down on the D-Pad. Moreover, the sword has a treasure-seeking ability called Dowsing. It can sense important people and items and help Link to locate them, acting as a sort of compass. Link can choose in the Dowsing menu which specific object or person he wishes to track down. The closer Link gets to the target, the faster the sword will beep.

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 Skyward Strike once it's swung. This beam can activate the various objects that the goddess specifically left for the hero, such as Goddess Cubes, Goddess Walls, and Goddess Crests.

Hyrule Warriors
In, the Goddess Sword appears as part of Fi's moveset. The three main tiers of the Sword are based on the upgraded versions of it that appear in Skyward Sword. While the first two tiers are named identically to their Skyward Sword counterparts, the Goddess White Sword was renamed the True Goddess Blade for its third tier. The star tier of the moveset added in the Master Quest pack DLC is the 8-bit Silver Arrow.

Theory

 * The presence of the Sages' emblems in the vestibule of the Sealed Temple (known in the past as the Temple of Hylia) is interpreted by some fans to mean that the ancient Sages crafted the Goddess Sword along with Hylia. This is potentially supported by a statement made by Princess Zelda in that the Master Sword was "crafted by the wisdom of the ancient sages."
 * 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.

Trivia

 * Despite the fact that evil beings cannot touch the Master Sword, Ghirahim is able to wield it both as the Goddess Sword and during the Thunder Dragon's Lightning Round; the former is likely due to the Goddess Sword's incomplete form, and the latter due to Lanayru's interference.
 * At the end of Skyward Sword, because of the time travel involved, the Goddess Sword and Master Sword exist simultaneously inside the Statue of the Goddess and the Temple of Hylia, respectively.
 * Despite being called the Goddess Longsword, it is in fact, an arming sword.