Susanoo

Susanoo is the third ability granted by the Mangekyō Sharingan, after Amaterasu and Tsukuyomi have been awakened in both eyes. 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 as well as physical being is 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, as the depletion of chakra likely places stress on the cells that partially produce chakra. Susanoo is formed through the materialization of the user's chakra and as such it may differ in appearance and colour between users. It begins by forming a large skeleton with muscle tissue around the user. Even in an imperfect state, the bones can act as armour for the user and it is capable of regenerating as long as the user has the energy to spare. Sasuke noted the tremendous strain of maintaining his Susanoo after using it for a prolonged time, and seemed to have accelerated Itachi's death from his illness.

Itachi's version
Itachi's version was first shown in an incomplete skeletal form that was reconstructing after saving himself from Sasuke's Kirin. It has been depicted as being crimson coloured. It is able to turn into a humanoid being complete with skin, armour and an actual face. It is further able to gain a layer of armour which resembles a yamabushi. In this final stage, it wields the sacred as a shield in its left hand and the legendary  in its right hand. The mirror is considered one of the three sacred treasures and is endowed with all nature transformations, it can change its own characteristics depending on the characteristics of the attack it receives, making the technique ineffective.

The sword, 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. Orochimaru sought the sword and Zetsu commented that he could never find it because it doesn't actually have a physical form. Zetsu also said that the Totsuka Sword combined with the Yata Mirror essentially makes Susanoo invincible, and it has thus been labelled the.

Sasuke's version
When Sasuke is first seen using Susanoo, he is only able to manifest its skeleton. Though useful as a defence, the Fourth Raikage proves to be able to snap the ribs or even bypass them and attack Sasuke directly; the Fifth Mizukage also managed to melt the bones with her Boil Release: Skilled Mist Technique. As his battle with the Raikage and his allies progresses, Sasuke begins further developing Susanoo as a large skeleton with some musculature on it, which envelops him more completely and includes giving it horns on its head and three hands that have six fingers on them. His version has been depicted as being purple in colour.

At this stage, Susanoo can use its fists or a chakra blade to attack but can still act as a defence for Sasuke. During his fight with Danzō Shimura, Sasuke is able to complete Susanoo, giving it the appearance of a demonic and evil-looking humanoid being with a bow that doubles as a weapon and shield. In this form, Susanoo can also produce chakra arrows from its right hands, which can travel at such speed that evading them is nearly impossible.

While fighting Kakashi Hatake and driven by hate, Sasuke's Susanoo gains a layer of armour, similar to Itachi's, but with jagged sharp teeth around the hooded area and with a much darker and menacing appearance. The orb in its third hand also changes to a pitch-black colour, while the bow in its left hand also took on a different shape. This form was seen only briefly, though, as Sasuke's deteriorating eyesight caused it to fade away.

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 both in appearance and composition compared to Itachi's Susanoo. Karin also noted how Sasuke's chakra had become "darker and colder" before and when he used this technique. It was also noted by Gaara, that it is the power of darkness.

Susanoo Evolution
Like a Jinchuriki's forms, Susanoo goes through many different stages before becoming complete, as we saw with both Sasuke and Itachi's usage of the technique. Before the technique is activated, their chakra begins to form into the Susanoo after they awoken both the Amaterasu and Tsukuyomi.

Version 1: Ribcage Form
The first form that Susanoo takes is the ribcage with a single skeletal arm after forming. Useful for a quick, mobile defence since it isn't as restricted as the larger forms. Most techniques aren't powerful enough to penetrate it-as we saw from Sasuke's version it was strong enough to hold up against Mei Terumī's Lava Release: Melting Apparition Technique and Chojuro's Hiramekarei Unleashing with no damage. However, sufficiently strong attacks, like A's Liger Bomb or Lightning Oppression Horizontal can shatter the defence by breaking the ribs, or Mei's Boil Release: Skilled Mist Technique can melt through it, leaving the user vulnerable.

Version 2: Muscle Form
The next form of Susanoo has the form grow to its complete size, gaining muscle over the bones for added defence. It also gains a sword which is strong enough to cut through multiple stone pillars with ease. The defence of this Susanoo form is great enough to block a barrage from Gaara's Successive Shots: Sand Drizzle, Kankuro's Red Secret Technique: Machinery Triangles, Temari's Scythe Weasel, and Darui's Storm Release: Laser Circus with little to no damage.

Version 3: Complete Skin Form
This is considered one of Susanoo's completed forms where it gains skin. In this form, it gains its main weapons-Susanoo Bow for Sasuke or the Sword of Totsuka and Yata Mirror for Itachi. This form's defenses are extremely high, and it is strong enough to anchor the user against the suction from a Baku (summon). However, like the previous forms sufficiently strong attacks can penetrate it, such as Danzo Shimura's Wind Release: Vacuum Serial Waves when powered up by the Baku's suction, and there seems to be a weak point at the base of the spine the enemy could exploit, which Danzo did.

Version 4: Complete Armoured Form
The final form of Susanoo gains its armor-which adds another layer of defence. It swallows the Skin Susanoo and makes it even more formidable. Sasuke's full version wasn't seen, but Itachi's was able to fight, and defeat Orochimaru's Eight Branches Technique. Like the previous forms, extremely strong attacks are able to defeat it-Kirin's firepower was able to destroy Itachi's, though it softened the impact so he could survive.

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.
 * The Totsuka Sword is the sword 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.
 * The is one of the, together with the  and the Yasakani no Magatama (八尺瓊曲玉) Jewel necklace.
 * 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.

Trivia

 * Prior to the completion of Sasuke's Susanoo, it was seen at least two times, once when Sasuke's "hatred" and chakra manifested as Susanoo before he attacked the samurai and the other time when its completed appearance was glimpsed as a shadowy figure in a genjutsu that Sasuke placed C under.
 * Susanoo's evolution is extremely similar to the Tailed Beast Cloaks that Jinchuriki gain when using their Tailed Beast Power. There are parallels between Naruto Uzumaki's Jinchūriki Forms and Sasuke's Susanoo with each stage of their development.
 * On Volume 51's front cover, Sasuke's Susanoo can be seen in the background and is shown to be bluish purple, in contrast to Itachi's being fiery red.
 * 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 jutsu).
 * 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, and it's more orange in colour than red from the anime. However, during his Awakening, Susanoo appears around him in a much smaller form.