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. At present, there are at least two people who can use the Mangekyo Sharingan: Itachi being one and Kakashi being the other. According to Itachi, if his younger brother Sasuke obtained it, the number of people who could handle the Mangekyo Sharingan (including himself) would be three. Itachi's statement may suggest that there is (or was at that time) another Mangekyo Sharingan user somewhere (because he didn't know of Kakashi's progress). However, Itachi uses the verb which means "to handle" or "to deal with", which could also mean that there are only three people left who are capable of acquiring the Mangekyo Sharingan. Given that Itachi had just killed every other member of the Uchiha clan, this statement could simply be taken as a matter of fact: Kakashi, Sasuke, and Itachi were the only known characters left alive with Sharingan eyes, so no one else had the potential to obtain it.

However Tobi has revealed himself to be Madara Uchiha, a likely candidate for the Mangekyo Sharingan meaning if Itachi knew about Madara, he didn't see Kakashi as being able to use the Mangekyo Sharingan as he wasn't a true Uchiha.

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 on the user.

It is also interesting to note that during the Return of Itachi arc in the anime, itachi uses the mangekyo several times, but the appearance of his sharingan don't change. It is not until the flashbacks during Sasuke Retrieval that it's actual form is shown.