Three Sword Style/Nine Sword Style

Kyutoryu is an addition to Zoro's main Santoryu style of swordsmanship, which uses, as the name suggests, "nine swords". Thus, Kyutoryu effectively triples Zoro's potential power.

Overview
Kyutoryu 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 a given foe at high speeds.

Zoro developed Kyutoryu before the two-year timeskip, before or during his intense duel with the CP9 agent Kaku. After being overpowered by Kaku's ferocious attacks in the latter's half-giraffe form, Zoro awakened his Kyutoryu, the use of which allowed him to overpower Kaku's most powerful Rankyaku and cut the CP9 member down. Afterwards, Zoro used his Kyutoryu one more time as a last resort against PX-4 on Sabaody, while badly wounded (as well as once more during the events of Movie 10).

Post-timeskip, while fighting on Onigashima's rooftop, Zoro used Kyutoryu a third time against the Emperor Kaidou, who was in his half-dragon form, and left another permanent scar on the Emperor. Kaidou himself speculated that Haoshoku Haki was used by Zoro to achieve said feat, but if Zoro's pre-timeskip use of Kyutoryu involved Haki is unknown.

Techniques
As with Zoro's other swordsmanship styles, Kyutoryu includes a number of derivative techniques that Zoro can use after having created his illusion of nine swords.


 * Kiki Kyutoryu: Ashura (鬼気 九刀流): The basis for Kyutoryu. 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), 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). The anime further adds to the first-seen activation of Asura: 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) 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 (弌霧銀): The first derivate Kyutoryu attack that is seen, used by Zoro to finish off Kaku. Zoro, holding his swords so that the blades are pointing toward the enemy, leaps toward them at great speeds, powerfully slashing them with all nine of his swords at once and ending 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—turning the massive slash wave into "mist" (a nod to the attack's name) before slicing up Kaku in his Human-Beast Form so thoroughly as to defeat him. The technique's name is a pun on ichibugin (一分銀), 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 (魔九閃): An attack similar to Ichibugin, where Zoro in his "Asura" 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 of his swordsmanship, including Kyutoryu. In addition to this, Kaidou has stated that Kyutoryu might incorpate Haoshoku Haki, though Zoro himself was unware of this, meaning that it might still be dormant.




 * Ashura Bakkei: Moja no Tawamure: A powerful Kyutoryu technique that may involve Haoshoku Haki, as speculated by Kaidou. 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 the "Asura" 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 blades, moves past his opponent in a flash, cutting them with incredible force. 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), a Kamigata Rakugo play from the Edo period. Instead of the usual "hakkei" (八景), said kanji are read "bakkei", with the name of this attack using "Bakkei" (抜剣) to play on the pronunciation. Unlike previous Kyutoryu uses, the kanji for "Asura" are accompanied by hiragana, not katakana. The Viz Manga calls the attack Asura: Blades Drawn Dead Man's Game.

Non-Canon



 * Ashura: Ugui: An attack only seen in Zoro's fight against Indigo in One Piece Film: Strong World. The move quite resembles Zoro's other Kyutoryu 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), the Japanese name of the big-scaled redfin, a species of fish. The movie's Funimation dub calls the attack Asura: Piercing Drill.

Trivia

 * 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 Kyutoryu 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). 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.
 * The concept of a sword attack involving nine simultaneous slashes, as seen with Zoro's Kyutoryu, is similar to the "Kuzuryūsen" technique used by Rurouni Kenshin's protagonist Kenshin Himura. Interestingly enough, Eiichiro Oda has worked as an assistant for the author of Rurouni Kenshin.
 * Kyutoryu is the fighting style that Zoro has gone the longest without using in the manga: 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).