Sharingan

Itachi's Sharingan

Being of the Uchiha clan, and as noted by Kakashi, Itachi was a true successor of the Sharingan^[6] mastering it already when he was eight years old. From a stationary position, he would study an opponent's movements with his Sharingan and respond accordingly, often with a genjutsu. He could cast a genjutsu by merely pointing at or looking at someone, and could similarly turn an opponent's genjutsu against them with Demonic Illusion: Mirror Heaven and Earth Change.^[7] Most of his genjutsu are derived from the Sharingan, but regardless of whether they are or not, all of them cause powerful mental trauma.
Using his Sharingan, he could also perfectly fake the suicide letter of Shisui Uchiha after he had killed him. He had trained so much with his eyes that he had reached the stage where he could have his Sharingan activated all the time, with minimal drain on his chakra levels.