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Not only is susano'o the third Technique, its the final technique of the mangekyou than can be awakened. According to the manga each eye respectively, contains:
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Pertaining to the kagatsuchi argument:
By administrating Keitai Henka (shape transformation) to Amaterasu, the user is able then able to manipulate its existence hence, determining the purpose of the technique. Itachi also was produce a world consisting of no limitations in which the user is able to place or remove physical entities like time, gravity, and space, are all controlled by the user. Hence, Itachi used Keitai Henka on Tsukuyomi.