The Kusanagi is a Light Blade that is found in the Debug Menu of the original Golden Sun. It is an Artifact that increases base Attack by 135. This weapon is impossible to access during normal play and can only be seen using a hacking device. It does not have an Unleash effect coded for it. This weapon's graphic is officially used in Golden Sun: The Lost Age for the Light Blade artifact Masamune. If it had been in the original game, it would have been the most powerful Light Blade, surpassing even the Kikuichimonji (assuming it would have had a proportionally strong Unleash effect coded as well).
If this weapon is in the party in a Clear Data file and the file is transferred into The Lost Age, it will be replaced with the Masamune.
The Kusanagi is a legendary Japanese sword as important to Japan's history as Excalibur is to Britain's, and is one of three Imperial Regalia of Japan. Kusanagi's high profile has made it popular, appearing in various works of fiction. It is rare for characters to actually use one in a combat as it is a ceremonial weapon. Instead, its magical properties are stressed. It is sometimes misrepresented as a katana, because it is a Japanese weapon. Kusanagi is allegedly kept at Atsuta shrine to this day, although it is not available for public display, and its existence cannot be confirmed.