The Dawnbringer is a legendary sword that was forged well before the time of ponies, even before Princess Celestia's birth. It is the brother sword of the Dawnbreaker.
The history of the Dawnbringer is scetchy at best. The earliest known wielder of the Dawnbringer was a Royal Knight of Princess Celestia about 1000 years ago, when King Sombra tried to overtake Canterlot with his dark magic. Using the Dawnbringer, the Royal Knight eventually defeated Sombra's magic and saved Equestria from his darkness. The latest known wielder of the sword is Lightblade, who found it in its dormant state as a blackened and rusted handle. Lightblade eventually took the blade to the temple inside the tallest mountain in the world, where he managed to restore the blade's power. He now wields it as his main weapon.
The Dawnbringer is described to be able to destroy any and all darkness absolutely. Any dark magic it comes to contact with is immediately destroyed, without a trace of it left. It is also able to ward off dark magic by outputting a field of energy that protects the user from coming into contact with any dark energy. Furthermore, the sword is completely indestructible from any means. It cannot be chipped, scratched, snapped, and it cannot rust or be tarnished by dirt or blood or anything else. The Dawnbringer, when it is not used for long periods of time, goes into a state of dormancy, when the blade disappears and the handle becomes prone to rust and wear and tear. There is only one place in the world that can restore the power of the Dawnbringer, being the temple inside the tallest mountain in the world. The sword also has the ability to sync with the user if the user has enough power, increasing its abilities exponentially. It has the ability to restore lost souls, including those destroyed by the Dawnbreaker, however this does not include bringing someone back to life.
The Dawnbringer takes the form of a shining silver colored longsword, its blade measuring 2.25 ft in length, and 3.5 cm wide. The crossguard takes a shape similar to angel wings, with a blue colored unknown gem in the middle of the hilt. The blade itself takes on a blue aura, leaving a blue trail of energy when moved. This energy field is the key to its darkness destroying ability, and the field itself cannot be broken or disrupted by anything other than its brother sword, the Dawnbreaker. When in perfect sync with the user, the blade turns white, and the aura is a shining platinum in color. The sword is made of an unknown metal that has been tested to be extremely light and indestructible by normal means.