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.

Susanoo Evolution
Susanoo goes through many different stages before becoming complete. Sasuke's development of Susanoo is marked by intense feelings of hatred. His Susanoo is also, 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.

First Stage: 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. While normal attack aren't powerful enough to penetrate it, sufficiently strong attacks can break through the defence leaving the user vulnerable.

Second Stage: Skeleton Form
The second stage Susanoo has the form grow to its complete size, gaining some muscle over the bones for added defence and destructive power. In addition, Sasuke's Susanoo also gains horns on its head and a chakra sword, which can cut through multiple stone pillars with ease. The defence of this form can block a barrage of attacks with little to no damage.

Third Stage: Complete Skin Form
This is considered one of Susanoo's completed forms where it gains skin. Itachi's and Sasuke's version begin to differ in appearance in this form: Itachi's looks like a warrior-like humanoid being, while Sasuke's looks like a evil and demonic humanoid being. In this form, it gains its main weapons- A Bow for Sasuke or the Sword of Totsuka and Yata Mirror for Itachi. This form's defences are extremely high, and it is strong enough to anchor the user against the suction of a Baku. However, like the previous forms, sufficiently strong attacks can penetrate it, and the back near the spine is weaker than the rest of the defence.

Fourth Stage: Complete Armoured Form
The final form of Susanoo gains its armour, which resembles a yamabushi and also adds another layer of defence. It swallows the Skin Susanoo and makes it even more formidable. Sasuke's version has a much darker and menacing appearance with more jagged sharp teeth around the hooded area. Like the previous forms, extremely strong attacks are able to defeat it.

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 once, when its completed appearance was glimpsed as a shadowy figure in a genjutsu that Sasuke placed C under.
 * 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.