After years of reading, I can barely recall the name Uchiha Shisui. From what I recall, he’s among the most powerful Sharingan users and Itachi’s best friend.
Chapter 550 shed some light on the shadowy shinobi that is Shisui. According to Bee, Shisui of the Shunshin (Shisui of the Body Flicker) is the most powerful genjutsu user of the Uchiha clan. Itachi affirmed this by saying that Shisui’s eyes can create the most power genjutsu that allows the user to gain complete control of the individual.
What is the Kotoamatsukami?
Kotoamatsukami (Distinguishing Heavenly God) is a Shinto term for the first powers that came into existence at the time of the creation of the universe. (source: Wiki)
In Naruto, Kotoamatsukami is the name of Shisui Uchiha’s ultimate Mangekyou Sharingan genjutsu. It allows the user to gain control over the target, without the target even realizing. Kotoamatsukami is so powerful that it cancelled the Edo Tensei (Impure World Ressurection) cast on Itachi.
Once activated, this technique requires 10 years before it can be used again.