Mangekyo Sharingan

The Mangekyō Sharingan (万華鏡写輪眼; Literally meaning "Kaleidoscope Copy Wheel Eye") is an advanced form of the Sharingan that has only been activated by a handful of Uchiha.

Overview

The Mangekyo Sharingan has the ability to control the Nine-Tailed Demon Fox, although Madara Uchiha is the only one to accomplish this milestone. It also grants them access to other powerful techniques.

The Mangekyo Sharingan is distinguished from a normal Sharingan by its appearance, which changes the form of the tomoe seal. Unlike the Sharingan, the appearance of the Mangekyo Sharingan differs from user to user; for example, Itachi Uchiha and Kakashi Hatake's resembles a pinwheel, and Sasuke Uchiha's looks like three intersecting ellipses.

The Mangekyō Sharingan is acquired through the death of the person closest to the Sharingan user. To expedite this occurrence, Uchiha throughout history have killed the person closest to them in order to meet this criteria, though how Kakashi and Shisui gained their respective Mangekyō Sharingan has yet to be explained. 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.

Over time, use of the Mangekyō Sharingan's techniques deteriorates the user's eyesight to the point of blindness. Only by taking and transplanting the eyes of another Uchiha clan member can the vision be restored, a process that results in the creation of an Eternal Mangekyō Sharingan (永遠の万華鏡写輪眼, Eien no Mangekyō Sharingan). The tighter the blood ties are between the "donor" and the receiver, the better it is for compatibility, siblings for example.

The time from fusing the donor's Mangekyō with the receiver's Mangekyō to the point of full recovery appears to be at least several days in the case of Sasuke. Not only does this process permanently restore one's eyesight, but it also gives the person even greater power. The Eternal Mangekyō Sharingan also seems to change form from the original, with what appears to be a fusion of the current owner's Mangekyō with the previous owner's.

source: [naruto.wikia.com]