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 may have a different appearance between the users, as Kakashi's version is significantly different than Itachi's. 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 does in fact do this to obtain his 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. It has also been implied in the manga that use of the Mangekyo Sharingan may deteriorate the user's normal eyesight.

In the long history of the Uchiha clan, only a few have been able to obtain the Mangekyo Sharingan, Madara Uchiha being the first. At present, there are three people who can use the Mangekyo Sharingan: Itachi, Kakashi, and Madara. According to Itachi, 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. Of course, this was before Kakashi got his.

Mangekyo Sharingan can release deadly jutsus, the only two known until now are Tsukiyomi 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. 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.

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 four 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.

Madara Uchiha
During Itachi's fight with Sasuke, Itachi tells Sasuke about his Mangekyo Sharingan and that he used its power to control the power of the Nine-Tailed Demon Fox. Madara is actually the first person to wield this dangerous weapon. Whether his power is due to a new Mangekyo Sharingan or if it's purely Madara Uchiha's power alone, it is unknown as of now.