A Mangekyō Sharingan (万華鏡写輪眼,Kaleidoscope Copy Wheel Eye) noted to be the "Heavenly eyes that see the truth of all of creation without obstruction (天壌の理を掌握せし瞳,tenjō no kotowari wo shōaku seshi hitomi)", is an advanced form of the Sharingan that has only been activated by a handful of Uchiha.
The Mangekyō Sharingan is awakened through the trauma experienced from the death of a person very close to the Sharingan user. With the user normally having to witness or experience this first hand, Uchiha throughout history have killed those closest to them in order to expedite this process. To obtain the Mangekyō Sharingan, one must feel the emotion of losing a friend or family, as merely killing them or watching them die will not activate it.
It is distinguished from a normal Sharingan by its appearance, which changes the form of the tomoe seal. Unlike the Sharingan, the appearance of the Mangekyō Sharingan differs from user to user with all known wielders of the dōjutsu having eyes that resemble variations of pinwheels; one variation is known as the "Straight Tomoe(直巴,Choku Tomoe)", which seems to grant the user fluidity in their movements when fighting.
The Mangekyō Sharingan drains a considerable amount of chakra and strains both the user's eyes and body, a drawback which affects a non-Uchiha even more so. Use of the dōjutsu gradually deteriorates the user's eyesight to the point of blindness, a harsh fate for those that have grown accustomed to the power and visual acuity it affords. Only by transplanting the eyes of another Uchiha can the user regain their sight and ocular powers. By transplanting the Mangekyō Sharingan of an Uchiha with strong blood ties to the recipient — a sibling - the Eternal Mangekyō Sharingan can be created. This process takes several days for a complete recovery, and not only permanently restores the recipient's eyesight, but also bolsters their own ocular power with that of the donor's, signified by the intertwined appearance of their Mangekyō Sharingan's tomoe seals. Due to the compatibility issues and the Mangekyō Sharingan's rarity, not everyone can gain these benefits by transplanting the eyes of just any Uchiha, which has been the cause of many fruitless sacrifices throughout the Uchiha clan's history. Thus far, only Madara and Sasuke are known to have been successful in this endeavor.
Madara and Sasuke were able to evolve their Eternal Mangekyō Sharingan into the Rinnegan Madara did so after implanting the DNA of Hashirama Senju into himself, though the dōjutsu only awakened years after the implant, when Madara's natural lifespan was almost over. Sasuke awakened his once Hagoromo gave him half of his power.