The prime lightsaber was a lightsaber constructed at the beginning of the Jedi Order's existence, during the Old Republic times. The weapon was destined for the Chosen One, and the very blade took a white glow.
The prime lightsaber was constructed at the beginning of the Jedi Order's existence as one destined for the Chosen One's usage, who was foretold to bring back the balance of the Force. The Chosen One, in the person of Anakin Skywalker, never found himself in possession of the artifact, though.
In 10 ABY, upon the command of Shmi Skywalker, the mother of Anakin, Kate began carrying out a range of analyses and explorations of the Jedi Order's archives. Everything she figured out was eventually passed to Leia Windu, who, in turn, informed her two relatives of these facts, Donata and Padme Kenobi. Either Leia or Kate, however, did not manage to report on this to Shmi, as they both died in the Attack on Rattoine.
Shmi Skywalker feared that the legacy of the prime lightsaber might be granted to the Chosen One's son, Luke, rather than to her, the Chosen One's mother. Thus, she aimed for destroying the artifact. Upon receiving information from the captured Luke, in 11 ABY she headed straight for Dantooine, to the Kenobis' family house. Once she had not come across anyone there, she decided to order her possession to be burned.
Soon after that, Dantooine was blown up by the cruiser Shmi's Eye, as well as Shmi was murdered and Leia Windu was brought back to life by Lanever Villecham. The woman passed everything she knew from Kate to Luke and Feel, the last Jedi knights. The men took it as their aim to find the legendary prime lightsaber.