Mangekyō Sharingan

The is a heightened form of Sharingan. The Mangekyo Sharingan is distinguished from a normal Sharingan through its appearance, which changes the form of the tomoe seal. However, unlike the Sharingan, it has a different appearance between the users, as a kaleidoscope never looks the same when looked through. Kakashi's version is significantly different than Itachi's, as Madara's looks completely different, and his brother's looks different also. Those who have the potential to obtain it must make a great sacrifice in order to obtain it – according to Itachi, they must kill their closest friend. Itachi, Madara, and his brother apparently do in fact do this to obtain the original version, but Kakashi has noted that everyone he cared about had died long before the start of the series, so how he developed his own variant is unknown. The major drawback of the Mangekyo Sharingan is that when it is first activated, it slowly begins to destroy the eyes, until the user becomes completely blind. The only way to regain one's eyesight after losing it would be to steal the eyes of a sibling, combining the two Sharingan to create a new, more powerful, and immortal Mangekyo Sharingan, which prevents blindness and apparently grants immortality.

In the long history of the Uchiha clan, only a few have been able to obtain the Mangekyo Sharingan, Madara Uchiha being the first, and his brother being the second. At present, there are three people who can use the Mangekyo Sharingan: Itachi, Kakashi, and Madara. According to Itachi though, if his younger brother Sasuke obtained it, the number of people who could handle the Mangekyo Sharingan (including himself) at that time would be three. What this means about Kakashi is unknown.

Mangekyo Sharingan can release 2 deadly jutsus, the only two known until now are Tsukuyomi and Amaterasu. Although these jutsu are extremely devastating, they require an enormous amount of chakra, and also cause immense strain and eventually blindness on the user.

Itachi Uchiha
Itachi Uchiha gained the Mangekyo Sharingan by killing his best friend, Shisui Uchiha. With the Mangekyo Sharingan, Itachi is able to use the two most powerful jutsu of the Uchiha clan. The first is Tsukuyomi. Upon making eye contact with an opponent, Itachi can use Tsukuyomi to transport them to an illusionary world where days amount to a mere moment in the real world. Because Itachi is in control of this world, he can torture an opponent in any way imaginable, leaving them crippled and in a state of mental collapse, though his limit is 3 days. The second jutsu is Amaterasu. While the jutsu has only been performed offscreen, it is shown to be capable of burning through the strongest of materials by creating a jet-black flame said to be as hot as the sun and capable of burning for seven days and seven nights. Both of these jutsu, however, use up massive amounts of chakra, Itachi himself becomes exhausted if using them both consecutively, and has to deactivate his Sharingan. Soon his ulterior motive is revealed, which would be to steal Sasuke's eyes to awaken his own Eternal Mangekyo Sharingan.

Kakashi Hatake
Kakashi Hatake developed his own version of the Mangekyo Sharingan during the timeskip. Kakashi uses his to create a localized interdimensional space-time warp, allowing him to target any object and send it to another dimension. The attack requires considerable effort to aim precisely and appears to leave Kakashi's chakra significantly drained; he collapses after using the technique three times in a single day (compared to his other original move, Lightning Blade, which he can use six times a day).

The appearance of the pupil and tomoe in Kakashi's Mangekyo Sharingan differ from that of Itachi's; Kakashi's looks more like a pinwheel. Before this was shown, it was thought that one had to kill their best friend to acquire the Mangekyo Sharingan. Given that Kakashi is unlikely to do this, due in part to his own admittance that everyone close to him is already dead, the means behind its development and the actual meaning of the "requirement" are currently unknown.

The difference in both look and abilities of Kakashi's and Itachi's Mangekyo Sharingan suggests that each Mangekyo Sharingan's look and abilities differ from one user to another.

Madara Uchiha
Madara Uchiha was actually the first person to wield this dangerous weapon. When he was young, he and his younger brother had the Mangekyo. He eventually grew blind from it, and possessed by the Mangekyo Sharingan he killed his younger brother and stole his eyes. This unlocked the "Eternal" Mangekyo Sharingan. Madara used its power to control the power of the Nine-Tailed Demon Fox and sent it to attack Konoha, but failed because of the Fourth Hokage's sacrifice. It is worth noting that Madara's tomoe seal changed after he stole his younger brother's eyes. The two unique tomoe seals seemed to have combined in Madara's new eyes.