The kekei genkei of the Uchiha clan is the sharingan. The sharingan is a dojutsu during the chunin exams Kakashi Hatake mentioned the rumor that the sharingan had evolved from the Byakugan. According to Tobi, the sharingan actually originates from the Sage of Six Path's elder son's dojutsu.
The sharingan has several abilities. Among them is the ability to see the color chakra as different colors. This abitlity also allows the user to tell if someone else is under a genjutsu by a disrupted flow of chakra. The sharingan can also see minute details and allows the user to predict movements.
Another ability of the sharingan is the ability to pick up on minute details and see fast moving objects. It allows the user to predict the movements of the opponent based on small actions.
In addition, the sharingan has the ability to cast weak genjustsus and break weak genjutsus used against the user.
The fourth and final ability is the ability to copy and mimic ninjutsu, genjutsu, and taijutsu that the opponent uses as long as the user has the capabilities to use them. For example the user cannot mimic an ice ninjutsu or any that requires more training in its area than the amount of training the user has.
There are several stages to the sharingan's developement
1. 1-tomoe: Able to see fast movements, through weak visual genjutsu (D-Rank) and predict most ninjutsu and taijutsu movements
2. 2-tomoe: Analyze faster movements, through slightly stronger visual genjutsu (C-rank), track handseals and mimic some actions
3. 3-tomeo: Analyze extremely fast movements, through moderate genjutsu (B-rank), mimic jutsus (nin, tai, and gen) exactly, and hypnotize targets with a lower rank than that off the user
4. Mangekyo Sharingan: Use unique techniques, see through stronger genjutsu (A-rank) - Eyes will bleed after use and the user will eventually go blind
5. Eternal Mangekyo Sharingan - achieved from the transplanting of another pair of mangekyo sharingan and the user will not go blind
The rinnegan was achieved by Madara after the Eternal Mangekyo but few details were given on how he did so and in the rp, this cannot be achieved.
The sharingan developes in steps 1 through 4 when the user suffers a traumatic experience.
Some users of the sharingan: Itachi, Shisui, Tobi, Izuna, Kakashi, Danzo, etc.
Non Uchiha users of the sharingan (Kakashi and Danzo) must always keep their sharingan activated and when in use, the sharingan consumes more chakra.
There are two special abilites also possesed by the sharingan but they result in blindness of the eye. They are Izanangi (which makes genjutsu a reality) and Izanami (which puts the target in a loop genjutsu until he/she makes the correct choice) Yhe conditions for activating them are not well known