Shusui, One Piece Wiki

Shusui is one of the 21 Great Grade swords and a "Black Blade" (黒刀, Kokutō?) that was once wielded by the legendary samurai Shimotsuki Ryuma. It is prized as the national treasure of the Wano Country. After his death, the katana was buried with Ryuma, until his grave was robbed by Gecko Moria 23 years ago. Ryuma appeared as a zombie on Thriller Bark, who had been given Brook's shadow. After his defeat, Ryuma bequeathed the sword to Roronoa Zoro, and said he was satisfied with Zoro as the sword's

One Piece: What is a Black Blade, explained

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