Sharingan is the blood inheritance limit of the Uchiha Clan. The Mangekyou Sharingan is a legendary doujutsu ability which has only emerged in a few clan members in its history. There is still mystery surrounding the eye and the Uchiha clan itself. Each user develops their own unique looking Mangekyou. The requirements for gaining the Mangekyou are very controversial in nature. After Uchiha Itachi murdered his best friend Shisui he gained Mangekyou. He told his younger brother Sasuke that to gain the Mangekyou, he too would have to kill his closest friend. Itachi told Sasuke that a scroll in their clan shrine revealed the true original purpose of the clan doujutsu. This scroll and Itachi himself revealed that the former leader of their clan, Uchiha Madara and his younger brother were the first to obtain Mangekyou. Madara himself did this by killing his friend, it's assumed his brother also killed a close comrade.
Usage of the Mangekyou techniques drains quite a bit of a user’s chakra, more so than any other normal technique. Also as a result of Mangekyou usage, the bearer will slowly lose their vision. Madara lost his vision before his brother and sought to regain his power. To do so he removed his brother's eyes and made them his own. With these new eyes, his Mangekyou took on a combined appearance of his original Mangekyou and his brother's. These "Eternal" Mangekyou eyes returned his vision and gave him unbelievable strength. With this strength it was said Madara used Sharingan's normal ability to supress the Kyuubi and tame it.