Shusui is one of the 21 O Wazamono grade swords and was once wielded by the legendary samurai Ryuma. It is considered the national treasure of the Wano Country. After his death, the katana was buried with Ryuma, until his grave was robbed by Moriah's crew.
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 master. The sword then replaced Zoro's Yubashiri which was destroyed at Enies Lobby.
Shusui's blade is black with a distinct red hamon and its tsuba hand-guard has flower-like edges, hence it is a "Black Sword". Its red hamon (hardening line) is Kanemoto style, having the appearance of a saw or sharp teeth. It has no accessories on the hilt, being wrapped with black silk. Kashira, tsuba and kojiri end cap are golden, and the sheath is black, decorated with dark red circles split in even thirds.
Shusui is designed to tremendously increase the damage potential and destructive power of the user. At one point, Zoro commented that Shusui's greatest attribute is its durability, claiming that even if a dinosaur were to step on it, the blade would not bend a millimeter. He clearly demonstrated it by blocking Oars' arm with the blade held horizontally. Furthermore, the sheer power of the blade alone was enough to redirect one of Oars' punches.
While it is capable of dealing heavy blows and is extremely hard, Shusui has a few major problems of its own. Just as Kitetsu III turned out to be a "problem child", Zoro describes Shusui as having a "temper" that makes it harder to wield. Zoro also said Shusui is quite a bit heavier than Yubashiri, which was extremely light, so it could be anywhere from normal weight to a good deal heavier than normal swords.
While in the hands of Ryuma, this sword was used by him in both life and death. As a zombie who was imbued with Brook's shadow, Ryuma had acquired the Fencing Style. Coupled with his already powerful skills, the modifications made to the zombie body, and the powerful attributes of Shusui, the Fencing Style had been revamped to the point of being completely different from Brook's original version.
When transferred into the hands of Roronoa Zoro, he used this sword to replace Yubashiri, which had been rusted by the Marine Captain Shu, as part of his Three Sword Style. When first fighting against Oars, Zoro discovered that Shusui's weight was the reason it was considered quite powerful. This was demonstrated by the fact that it created a larger vacuum when the 108 Caliber Phoenix was performed, and absorbed the vacuums from his two other swords, making its already great destructive power increase tremendously, as Zoro puts it. This weight contributes to the decreased control Zoro has over some of his attacks. As time passed, Zoro began to master Shusui's negative attributes.
After his training with Dracule Mihawk, this sword seems to have become Zoro's favored blade, as he has been using it frequently with his One Sword Style techniques, when prior to the time-skip he mainly used the Wado Ichimonji, the sword he inherited from Kuina. To prove his mastery over Shusui after the time-skip, Zoro was seen easily cutting a ship in half with it shortly before reuniting with his crew. Furthermore, the majority of opponents that Zoro has faced since leaving Sabaody Archipelago, were dealt with One Sword Style techniques while using Shusui.
The sword originally belonged to Ryuma, and was buried with its original master in Wano Country upon his death, where it was regarded as a national treasure. Sometime after that, both the sword and Ryuma's body were stolen, and Wano Country were brought to its knees in shock.
Ryuma wielded Shusui once again when he was revived as a zombie, and later bequeathed it to Zoro upon his defeat.
Upon seeing it, Tashigi was awed, while Kin'emon became determined to challenge Zoro so the sword can be returned to Wano Country.