Using the completed form of Susanoo, Itachi creates the Yasaka no Magatama with its right arms, consisting from three tomoe bound together through their centre’s by a single circular chakra thread. Due to this construction, the tool acts in a similar manner to that of a shuriken when thrown by Susanoo, rapidly spinning towards the intended opponent until its ultimate collision. Itachi asserts that this is his strongest long-range technique.
The Yasakani no Magatama (八尺瓊曲玉; Literally meaning “Eight Shaku Curved Jewel”) is one of the Three Imperial Regalia of Japan (三種の神器, Sanshu no Jingi), together with the Kusanagi no Tsurugi (草薙劍; Literally meaning “Grass Cutter Sword”) and the Yata no Kagami (八咫鏡; Literally meaning “Eight Span Mirror”).
Source: Naruto Wikia
