Sharingan is one of the greatest eye techniques in naruto world the other are rinnegan and byakugan. Sharingan power is the bloodline limit and hidden within every members of Uchiha clan. When the user is in life threatening event sharingan will automatically activate itself to safe the user from dangerous situation. After the first activation the user will be able to use the power anytime at their will.
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The first member of Uchiha clan to ever used sharingan is Madara Uchiha, he is also one of the founder of Konoha village along with Shodai the first Hokage. Uchiha clan are well known for their elite combat skills and fire manipulation techniques ability.
Sharingan doujutsu have the ability to read, analyse and copy all other jutsu such as genjutsu, taijutsu and ninjutsu except bloodline limit ability. The user can perfectly mimic normal jutsu and can also predicted future movement eventhough the user opponents didn't perform that movement at all. The advantage of sharingan user is that they have the ability to read and destroy any kind of genjutsu if they are stronger and have more skills than the opponents that cast genjutsu.