The Lunar Saber is a rare, ancient white-bladed lightsaber and it's only user is Princess Yuna.
Unlike other lightsabers the Lunar Saber's blade is flat, thin, and curved to a point more typical of a metallic sword. It's blade glows as bright as a full moon leaving a faint white, electrical aura along its edges, it hummed with a higher-pitched whine than its more common lightsaber counterparts when the blade was active, and made a sound like a whistle when it was swung. And the hilt of the saber has a black glow on it.
The Lunar Saber was created by the first Moon Spirit 275 years ago and it was used in the Moon Spirit's many battles. The Lunar Saber was later used by many of the older Moon Spirits. Until it was lost in the tomb of the Moon Spirits. But it was soon found by Princess Yuna, the daughter of Princess Luna. So she then took the saber and now weilds it, along with her Lightsaber.
She then first used the Lunar Saber in a duel against Nightmare Moon. But was defeated and then Hiccup used the Lunar Saber to bisect Nightmare Moon