Susanoo

Susanoo is the third ability granted by the Mangekyō Sharingan, after the ability in both eyes have been awakened. It creates a gigantic, spiritual humanoid being that surrounds and protects the user. According to Madara Uchiha, for a Sharingan user to activate the Susanoo is a rarity. Like the other techniques granted by the Mangekyō Sharingan, the user's eyes and body are strained while using Susanoo because it consumes a large amount of their chakra. Sasuke described it as feeling pain in every cell in his body which only grew from prolonged use. Susanoo is formed through the materialisation of the user's chakra and as such differs in appearance as well as colour between users.

Development
Susanoo goes through several development stages as users gain control of it. The various stages are best seen while Sasuke is learning how to use Susanoo, though Itachi can briefly be seen with both incomplete and complete forms when he first begins reforming Susanoo after Kirin hit it and when he runs out of chakra and nears death. In its earliest form, users can only manifest some of Susanoo's skeleton. Sasuke does this to produce a ribcage that he uses as a defence, though the bones can be broken, melted, or shattered. He also makes an arm to interact with his surroundings on one occasion. Because these skeletal structures do not entirely surround them, users can move around much easier.

As users gain additional control, musculature and skin begins to appear on the bones. It grows to its full size and the user is completely surrounded, eliminating the vulnerabilities of the skeletal form. However, Susanoo can also appear behind the user instead of just surrounding them. The user can also change the size of it to fit the current situation. Susanoo eventually develops into a "complete" warrior-like form, with skin, armour and its final arsenal of weapons finally manifesting. It can anchor the user to the ground at this stage, preventing opponents from forcibly moving them unless they can get through Susanoo's various layers. In its final form, this fully humanoid form is surrounded by a secondary layer of armour that causes Susanoo to resemble a yamabushi.

Itachi's Version
Itachi's version is crimson coloured (its complete form, however is dark orange coloured). In its left hand it wields the Yata Mirror as a shield. The mirror is endowed with all nature transformations, allowing it to change its own characteristics depending on the the attack it receives, thus nullifying the attack. In its right hand it wields the Sword of Totsuka. The sword is a kind of Sword of Kusanagi also known as the, is sheathed in a sake jar held by a third hand growing from Susanoo's right forearm. It is an ethereal weapon with an enchanted blade capable of sealing anything it pierces. Victims of the sword are trapped in a genjutsu-like "world of drunken dreams" for all eternity. For longer-ranged attacks, it is able to use the powerful Yasaka Magatama, which consists of three chakra tomoe bound together by a single circular chakra thread. Zetsu said that the Totsuka Sword combined with the Yata Mirror essentially makes Susanoo invincible, and it has thus been labelled the. Unlike Sasuke whose Susanoo grows up in form and power via Sasuke's hatred, Itachi does not seem to need that to control his Susanoo's forms.

Sasuke's Version
Sasuke's version is purple in colour. Once he masters it, it gains a bow that doubles as a weapon and shield. Susanoo produces chakra arrows, via an orb held by its third hand, which can be fired at such speed that evading them is nearly impossible. It can also use the arrows as a makeshift melee weapon and after developing into its armoured form, the orb and arrows become pitch black.

Sasuke's development of Susanoo is marked by intense feelings of hatred; whenever angered by the words or actions of his opponents, Susanoo grows in power. His Susanoo is, accordingly, noted to be much darker in composition compared to Itachi's Susanoo. In both its incomplete and complete forms, it has a more demonic appearance in contrast to Itachi's, which have a more "normal" human-like appearance; possessing horns, and a malevolent grin. In its final form, it also possesses jagged teeth around its hood (unlike Itachi's which feature straight teeth) and magatama earrings.

After Sasuke gains the Eternal Mangekyō Sharingan, the second layer of armour changes its appearance and it seems to be lighter in colour in comparison to its previous forms. The mouth of the armour changes to a beak shape, giving it an even more menacing look. The skin on the right arm and right hand starts peeling and the armour itself appears more ghastly in appearance. It seems that the arrows are now coated in the flames of Amaterasu as seen when one of them pierced a White Zetsu clone and set him ablaze.

Influence

 * Similar to other Mangekyō Sharingan abilities, Susanoo was taken from Japanese mythology. Susanoo, the god of sea and storms, was the brother of Amaterasu and Tsukuyomi, with Susanoo born from Izanagi washing his nose.
 * Susanoo's designs are based on, well known yokai that are associated with the ascetic practice of shugendō. In their final forms, both incarnations of Susanoo are garbed in the attire of shugendō practioners, known as yamabushi, and wear hoods that resemble tengu masks, with Itachi's appearing as a long-nosed hanataka tengu and Sasuke's as a crow-billed karasu tengu. Susanoo-no-Mikoto is said to have been the progenitor of the tengu.
 * The is the sword used by Izanagi to kill his newborn son, Kagu-Tsuchi, after his birth burned his mother, Izanami, to death. From the blood of the Totsuka no Tsurugi, eight new Shinto gods were born. The same sword is later used by the god Susanoo to slay the eight-headed and eight-tailed snake, Yamata no Orochi, just like it was used by Itachi's Susanoo to defeat Orochimaru's.
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Trivia

 * In the anime, the final form of Sasuke Uchiha's Susanoo is portrayed differently: aside from five fingers instead of six, and having straight teeth, it was shown wielding the gourd that houses the Totsuka Sword in the secondary right hand and the bow with no orb in the primary right hand. The gourd is in the same colour as Sasuke's Susanoo, as opposed to Itachi's.
 * Prior to the completion of Sasuke's Susanoo, it was glimpsed as a shadowy figure in a genjutsu that Sasuke placed C under.
 * Both Itachi's and Sasuke's final form of Susanoo's mouth area always appear to face to the right but the mouth area of Sasuke's new version faces to the left side instead.
 * Sasuke's bow-wielding form of Susanoo bears a great resemblance to Raideen the Brave, one of a number of Yoshiyuki Tomino references in the series (another being the Gundam-derived names of Kakuzu's techniques).
 * The Totsuka Sword wielded by Itachi's Susanoo is considered to be a "Sword of Kusanagi", although this has been popularly mistranslated as it being the perfect counter to the Sword of Kusanagi.
 * During the Sasuke vs. Itachi boss battle in Naruto Shippūden: Ultimate Ninja Storm 2, Susanoo doesn't appear around Itachi, but behind him. However, during his "awakening", Susanoo appears around him in a much smaller form.
 * When it was first shown, Itachi's Susanoo had five fingers on its hands. Recently however, it has a total of six fingers per hand instead of just five just like Sasuke's.