Nine Sword Style is an extension of Zoro's spirit and, as the name suggests, involves nine swords. Through sheer will, Zoro creates an illusion of two more heads and four more arms appearing on him, which wield copies of his swords for a total of nine blades. At the same time, a three-headed, six-armed silhouette appears behind Zoro, making him resemble a "demon god" (the mythological asura). Thus, Zoro's power grows threefold, as he can use his nine swords to slice an opponent thrice more than usually, and Zoro tends to do so by rushing past his foe at high speeds.
Zoro developed Nine Sword Style before the two-year timeskip, first showing use of it during his intense duel with the CP9 agent Kaku. After being overpowered by Kaku's ferocious attacks, Zoro awakened Nine Sword Style, which allowed him to overpower Kaku's most powerful Rankyaku and cut the CP9 member down. Afterwards, Zoro used his Nine Sword Style one more time as a last resort against PX-4 on Sabaody, while badly wounded, plus once more during the events of Movie 10.
Post-timeskip, while fighting on Onigashima's rooftop, Zoro used Nine Sword Style a third time against the EmperorKaidou, who was in his half-dragon form, and left a permanent scar on the Emperor. Kaidou himself noted that Haoshoku Haki was used by Zoro to achieve said feat, although it is unknown if Zoro's pre-timeskip usage of Nine Sword Style involved Haki.
As with Zoro's other swordsmanship styles, Nine Sword Style includes a number of derivative techniques that Zoro can use after having created his demonic illusion of nine swords.
Kiki Kyutoryu: Ashura (鬼気 九刀流 阿修羅,Kiki Kyūtōryū: Ashura?, literally meaning "Ghastly Nine Sword/Blade Style: Asura"): The basis for Nine Sword Style. Zoro, with his Wado Ichimonji in his mouth and his other two blades held out in front of him, creates an illusion born of his "spirit" (気迫,kihaku?), according to Kaku. To onlookers, it appears as if Zoro gains an additional pair of heads and four more arms, all wielding duplicates of his swords—each appearing twice more, for nine swords total. At the same time, an indistinct, six-armed, asura-like silhouette forms behind Zoro, the same entity he comes to resemble, described by Kaku as a "three-faced, six-armed demon god" (三面六手の鬼神,sanmen-rokushu no kishin?). The anime further adds to the first-seen activation of Ashura: Zoro, angered by Kaku, had a dark aura begin surrounding him, expanding several meters around where he stood and causing a swirling gust to form.Ashura (阿修羅,Ashura?) is the Japanese term for "asura", a class of demons/demigods from Hindu and Buddhist mythology. In the VIZ manga, the technique is called Demon Aura Nine-Sword Style: Asura and, in the Funimation sub and dub, the name is Demon Aura Nine-Sword Style Asura.
Before the Timeskip
Ashura: Ichibugin (阿修羅 弌霧銀,Ashura: Ichibugin?, literally meaning "Asura: One Mist Silver"): The first derivate Nine Sword Style attack that is seen. Zoro, holding his swords so that the blades are pointing toward the enemy, leaps toward them at great speeds, powerfully slashes them with all nine of his swords at once, and ends up behind the enemy's back, holding his blades in a manner that makes them look fanned out. The attack was strong enough to disperse Kaku's Rankyaku "Amane Dachi"—his strongest attack—by turning the massive slash wave into "mist" (hence the attack's name) before slicing up Kaku in his half-giraffe form, finishing him off. The technique's name is a pun on ichibugin (一分銀,ichibugin?), meaning "a silver quarter ryō". In the VIZ Manga and Funimation dub, the attack is called Asura: Silver Mist, and in the English dub of the game Unlimited Cruise it is called Asura Ichibugin. In the Wii video games, the Unlimited series, and Gigant Battle, this move does not see Zoro charging against the opponent, instead producing a longer-ranged, expanding shockwave.
Ashura: Makyusen (阿修羅 魔九閃,Ashura: Makyūsen?, literally meaning "Asura: Demonic Nine Flashes"): An attack similar to Ichibugin, where Zoro in his "Ashura" state approaches the enemy to consequently slice them up with his nine swords, before ending up in a stance that has the blades resemble a wheel shape. The attack was first seen in use against the Pacifista PX-4, breaking through its harder-than-steel armoring to draw blood from the cyborg. In the anime, the game Unlimited Cruise, and the Pirate Warriors games, the move has Zoro perform a rotatory motion that is not observed in the manga. In the VIZ manga, the technique is called Demon Nine Flashes while in the in the Funimation sub of the anime, it is called Asura's Nine Lightning Strikes.
After the Timeskip
During his two-year training period with Mihawk, Zoro learned to imbue his sword with Busoshoku Haki, drastically increasing the power behind his sword attacks, including Nine Sword Style. In addition to this, during their battle, Kaidou noticed that Zoro unintentionally incorporated Haoshoku Haki into his Nine Sword Style.
Ashura Bakkei: Moja no Tawamure (阿修羅抜剣亡者戯,Ashura Bakkei: Mōja no Tawamure?, literally meaning "Asura Sword-Drawing: Jest of the Dead"): Zoro holds Enma out before him, horizontally in his right hand, his other swords held in his left hand and mouth as usual, and enters his "Ashura" state, with the silhouette behind him clasping two of its six hands. Haki begins emanating from Zoro as he imbues his blades with it, turning them black. He then, readying his swords, moves past his opponent in a flash, cutting them with incredible force. Additionally, as noted in shock by Kaidou, Haoshoku Haki appeared to have been used in the attack as well, although Zoro did not know this as he was not even aware he possessed this Haki. The attack was first used on Kaidou, with Zoro putting all his remaining strength into the slash. It proved strong enough to significantly injure the Emperor, giving him a large, bloody gash across his front torso; Kaidou, while remaining on his feet, acknowledged that the blow would leave a scar (which would make it the second technique ever to give Kaidou a permanant scar). The attack's name references Jigoku-bakkei Mōja no Tawamure (地獄八景亡者戯,Jigoku-bakkei Mōja no Tawamure? "The Eight Scenes of Hell, Jest of the Dead"), a Kamigata Rakugo play from the Edo period. Instead of the usual hakkei (八景,hakkei?), said kanji are read bakkei, with the name of this attack using Bakkei (抜剣,Bakkei? literally meaning "Drawing One's Sword", though correctly read Bakken) to play on the pronunciation. Unlike previous Nine Sword Style uses, the kanji for "Asura" are accompanied by hiragana, not katakana. The VIZ Manga calls the attack Asura: Blades Drawn Dead Man's Game.
Ashura: Ugui (阿修羅穿威,Ashura: Ugui?, literally meaning "Asura: Piercing Might"): An attack only seen in Zoro's fight against Indigo in One Piece Film: Strong World. The move quite resembles Zoro's other Nine Sword Style attacks, the main difference being that, as Zoro rushes past his enemy, violently slicing them with his illusionary nine blades, said enemy is blown up high into the air from the impact. Using this technique, Zoro defeated Indigo in a single blow, despite the scientist having sustained no prior injury in their fight. The technique's name likely comes from ugui (石斑魚,ugui?), the Japanese name of the big-scaled redfin, a species of fish. The movie's Funimation dub calls the attack Asura: Piercing Drill.
The "asuras" (Sanskrit: असुर) are a class of powerful, power-hungry demigods—often portrayed as demonic with three heads and six arms—that appear in Indian religions. They are featured in many Buddhist and Hindu cosmological theories and, in both Buddhism and Hinduism, are prominently described as battling the more benevolent, heavenly "devas" or "suras" (gods), being considered the "enemy of the gods" in Vedic and post-Vedic literature. More specifically in Buddhism, asuras ("antigods", "demigods" or "titans") constitute the third of the six Saṃsāric realms of rebirth and existence that reincarnation into occurs based on karma gathered in current/past lives. Into the realm of asuras, people are reborn due to actions based on jealousy, struggle, combat, or rationalization in a previous life.
Thus, Zoro's Nine Sword Style follows the theme of his other sword techniques that often incorporate Buddhist terminology/imagery, as well as "demonic" motifs.
Upon first seeing the illusion of Zoro with three heads and six arms, with the figure behind him, Kaku described what he saw as a "demon god" (鬼神,kishin?). This term, which most literally translates as "demon/oni god", is usually used to refer to ""fierce or wrathful deities" in Buddhist myth (wrathful "aspects" of enlightened buddhas, bodhisattvas, or devas), but is also applicable to more "demonic" beings with superhuman power/ability, such as asuras or rakshasas, as well as "fierce" Shinto gods.
Nine Sword Style is the fighting style that Zoro has gone the longest without using in the manga (without considering the No Sword Style, that has not been used again since its reveal back in 2004): in fact, between its last use pre-timeskip and its first use post-timeskip, 12 years and 8 months passed in real life (two years in the story's timeline).
↑ 1.01.11.2One Piece Manga and Anime — Vol. 43Chapter 417 (p. 16-17) and Episode 300, As Zoro prepares his Nine Sword Style to use against Kaku, the latter recognizes the vision he sees as a product of Zoro's spirit.