Mangekyō Sharingan

The most powerful form of the Sharingan, the Mangekyou Sharingan (translated to the Kadieoscope Sharingan in some translations), although it is not a natural state of the sharingan, once it is activated, it can release deadly jutsus, the only two known to date are Tsukiyomi and Amaterasu. Although these jutsu are extremely devastating, it requires an enormous amount of chakra, and also it causes immense strain on the user.

For an Uchiha to activate the Mangekyou Sharingan he must kill his best friend

There is currently 3 users who have successfully activated it:
Uchiha Itachi who killed his best friend Uchiha Shisui Hatake Kakashi some theories suggest that because his best friend Uchiha Obito died for him, he was able to activate it Uchiha Madara although we have not seen his mangekyou sharingan, it is safe to assume he has obtained it long ago