Kokuyōseki no Ha

The Kokuyōseki no ha (黒曜石の刃, viz. Blade of Obsidian) is a blade wielded by Kane Soga. He found the sword in a shrine located on the outskirts of Sunagakure,infront of the blade laid a scroll which read "Strongest blade in existence, only those who have strong chakra force can weild it." Also it tells a legend of how strong the blade is, as far as saying it cannot be broken.