The title say it all.Itachi had read the uchiha tablet containing alot of info of his sharingan before he left Konoha.Itachi did also have MS when he killed the uchiha clan and also shusi give Itachi one of his eye during that time.We all knew that itachi was specialise in genjutsu ,he got the legendary item and very knowledgeable ninja.
Izanagi
Itachi could have used izanagi on himself when he got sick with the unknown disease and restore his body to it orignal self.After using Itachi used izanagi he could replace the blind eyes with shusi eye since the shusi's eye was specialise in gensjutsu and also contain the strongest gensjutsu in world.We know that itachi will no perform susano but he will be able to perform tsukyomi , KA and also have the lengendary items of so6p to compensate for susano because the totsuka sword and yata mirror can changes sizes .Itachi will still be invincible ninja in the series
Izanagi
Izanagi is a genjutsu that is cast on the user instead of others and is the most powerful amongst this type of genjutsu.[1] When activated, the caster removes the boundaries between reality and illusion within their personal space. To a degree this allows the user to control their own state of existence, but it is normally only active for the briefest of moments.[2]
While the user remains physically real while fighting, this technique is capable of turning any occurrence including injuries and even death inflicted upon themselves while the technique is active into mere "illusions". Whenever the user receives a fatal injury, he or she automatically fades away as though they were an illusion all along and then returns back to reality; physically real and unscathed. The technique is based on an ability the Sage of the Six Paths had, referred to as "Creation of All Things" (万物創造, Banbutsu Sōzō). The process he used is explained to have initially involved the administration of imagination, and the spiritual energy which forms the basis of Yin chakra to create shape and form from nothingness. Then, through the application of vitality, and the physical energy which forms the basis of Yang chakra, he would breathe life into the prior form. Thus, a technique with the power to turn imagination into reality was born, known as Izanagi.[3]
While the user remains physically real while fighting, this technique is capable of turning any occurrence including injuries and even death inflicted upon themselves while the technique is active into mere "illusions". Whenever the user receives a fatal injury, he or she automatically fades away as though they were an illusion all along and then returns back to reality; physically real and unscathed. The technique is based on an ability the Sage of the Six Paths had, referred to as "Creation of All Things" (万物創造, Banbutsu Sōzō). The process he used is explained to have initially involved the administration of imagination, and the spiritual energy which forms the basis of Yin chakra to create shape and form from nothingness. Then, through the application of vitality, and the physical energy which forms the basis of Yang chakra, he would breathe life into the prior form. Thus, a technique with the power to turn imagination into reality was born, known as Izanagi.[3]