Sun Blade

The Sun Blade (日輪刀, Nichirin Katana) is the secret blade made on an ancient continent which sank into the ocean.

The blade was kept by Nichirin and other swordsmasters for generation. Nichirin hid the blade in the pocket dimension in his pearl necklace and gave it to Atomic Samurai before passing away in the battle against the Monster Association. It has a similar weapon known as the Moon Blade, as it's opposite.

The legend says that the warrior who in the possession of the Sun Blade and the Moon Blade together will obtain unparalleled power. Bringing that legend to reality is the Council of Swordmasters's longstanding mission.

Trivia

 * The Sun Blade is also known as "The Nichirin Sword"