Itachi is one of the most powerful and influential characters in all of Naruto. In addition to that, he is quite a strong character who has one of the best Susanoo. The above is mainly due to the Mirror of Yata and the Totsuka Sword.
The origin of the Mirror of Yata and the Totsuka Sword
The origin of this shield and sword originates in Japan. The sword is said to be the sword used by the god Susanoo to kill the 8-headed snake called Yamata-no-orochi.
The above can be evidenced in the anime when Itachi fights Orochimaru. In the anime, the Totsuka Sword is a disembodied weapon, that is, it is not made of physical materials. So it is a spiritual weapon that has the ability to cut anything, but, this weapon is famous especially for sealing in a very powerful Genjutsu to all those objects or people that it cuts with its edge.
For its part, the Mirror of Yata is a magical shield, which also has no incorporeal character. It is known for providing its bearer with an impenetrable defense, since it is capable of nullifying any attack of any nature it receives.
These two weapons in combination are capable of providing the perfect defense to the Susanoo’s bearer, in this case Itachi Uchiha. But how did he get these powers? No one really knows.
In the series they never explained this, but, it is inferred that Itachi’s Susanoo was developed to such an extent that Itachi himself obtained the swords automatically over time. It is believed that Itachi obtained both weapons when he obtained the Susanoo God’s blessing, although this was never confirmed.
Whether his origin is known or not, the fact remains that Itachi was one of the best Susanoo carriers in all of Naruto.