Sharingan

Overview
The Sharingan is the dōjutsu kekkei genkai of the Uchiha clan that appears selectively among its members. It is regarded as one of the "Three Great Dōjutsu", the others being the Byakugan and the Seikatsugan. While its powers originated from The Great Hybodus's Eyes, its independent form was first manifested by Takumi Uchiha.

Acquisition
When a wielder of this kekkei genkai experiences a powerful emotional condition with regards to a person precious to them, their brain releases a special form of chakra that affects the optic nerves, transforming the eyes into Sharingan; for that reason the Sharingan is described as an "eye that reflects the heart". Often, as per the Uchiha's so-called "Curse of Hatred", this emotion is a negative one, brought on by stress or loss. The emotion can also be positive, driven by a desire to protect or reunite with a loved one.

As with any dōjutsu, Sharingan can be transplanted into non-Uchiha. They are unable to deactivate it and its use has a greater chakra demand for them than for actual Uchiha, who need only minimal chakra to keep it active. For these reasons, non-Uchiha tend to cover their Sharingan when they aren't using it in order to conserve chakra. Sharingan that originate from the same Uchiha are still linked to some extent even when wielded by separate users, at times enabling those users to see the same things.

Abilities
The Sharingan grants the wielder two broad abilities: the "Eye of Insight" and the "Eye of Hypnotism". Although the abilities are not literally restricted to certain eyes, Takumi Uchiha noted that the Sharingan can only be used to its full potential when both eyes are together. Even within these two categories, there may exist several distinct though similar abilities:

"Eye of Insight" "Eye of Hypnotism" In addition to these abilities there are two other jutsu that a Sharingan user can perform: Izanagi allows the user to warp reality for a short time, changing reality into illusion and illusion into reality, thus escaping death and rewriting destiny; Izanami traps the victim in an infinite loop disregarding the target's five senses, specifically designed as a reprimand for those who would abuse Izanagi. Use of either ability causes the Sharingan with which it is performed to be blinded afterwards, and for that reason the Uchiha classified them as kinjutsu.
 * 1) The user can see chakra, giving it colour in order to distinguish it by its composition and source. Though not as capable in this regard as the Byakugan, it can see chakra through some (but not all) obstructions and detect irregularities in a person's chakra flow, such as those caused by genjutsu influence.
 * 2) The user is granted incredible clarity of perception, enabling them to read lips or mimic something like pencil movements. Kagami Uchiha's Sharingan was even able to see on a cellular level. In combat, this allows them to see fast-moving objects and, once fully developed, offers some amount of predictive capabilities: they can anticipate an opponent's next move based on the slightest muscle tension in their body and act accordingly to dodge or intercept. They can also read the enemy's hand seals to give them an insight of the performed technique's nature, regardless of the speed of performance, so long as the hands are not physically hidden from view. Although a Sharingan user can see these things, they also need the physical ability to actually act on the visual information. It is shown that with advanced enough prowess, via eye contact, the user can even enter the target's mind to look through their memories, and even erase them within a long range for several targets.
 * 3) The user is able to copy almost any jutsu they see, memorising ninjutsu, genjutsu, and taijutsu with near-perfect accuracy. They can then either perform that jutsu or modify it to suit their needs, as when Kagami Uchiha bases his Inferno Style: Shadow Shuriken Flames on Kenji Nara's Shadow Strangle Technique. Again, Sharingan users need the prerequisite abilities before they can mimic a jutsu they have seen, and for that reason they cannot, for example, reproduce kekkei genkai abilities they do not have or nature transformations they haven't learned to perform
 * 1) The user can perform what is generically called Genjutsu: Sharingan. While there are various manifestations and purposes of the ability, the better known one is to, through eye contact, suggest thoughts and actions to a target, coercing them to divulge information or act in a particular way, potentially without the target's awareness. In extreme cases, the Sharingan user can take complete control of the target's body, forcing them to do exactly as the controller wishes; this control is visually indicated by the Sharingan's design appearing in the target's eye. The Uchiha are particularly infamous for using this ability on the tailed beasts, and in fact can enter its or a jinchūriki's subconscious to more precisely control the beast's chakra. Multiple targets can be taken control of in this way, though the more divided the user's attention is, the easier it is for targets to escape this control.
 * 2) While not an ability per see, the Sharingan user can appear as if they are able to "see the future". First, the user must perfectly mimic every move their opponent makes and possibly their thoughts and words through careful observation. Then, they suggest an action to the opponent such as hand seals for a technique that the opponent and Sharingan user already know and copy that. With these things done, the user can complete the action even before the opponent does, possibly using the Sharingan's genjutsu to confuse or delay them.