Genjutsu: Sharingan is simply the generic use of genjutsu as employed by Sharingan wielders. By establishing direct eye contact with the intended opponents, the user can lock them within a genjutsu of their choosing in order to achieve various effects. These effects range from inducing instantaneous, but temporary, paralysis or unconsciousness in an opponent to forcefully extracting any valuable information that they possess or controlling their actions.
Using his Sharingan, Sasuke stares at his opponent directly in the eyes and they stare back. Then a shadowy figure of himself appears before them forcing them under a genjutsu. An image resembling his Susanoo appears in front of the opponent and grasps them as it begins "assimilating" them into itself. This breaks the opponent's will. Afterwards, the opponent is left incapacitated.
Itachi is seen using an illusion that traps the victim in a dark empty space where their body is made of paper that burns from toe to head while the user is watching. The genjutsu will make it harder for the target to move properly. It bears a great resemblance to his Tsukuyomi, though it is used with the regular Sharingan.
When Sasuke used this genjutsu, he was seen covering his left eye in apparent pain afterwards. Sasuke's completed version of Susanoo was first seen in this genjutsu, though as a shadowy figure before its complete début in the fight against Danzō.