Eternal Mangekyo Sharingan

The Eternal Mangekyo Sharingan is a form of Sharingan that after the user goes blind from using Mangekyo Sharingan and steals a siblings Mangekyo Sharingan eye(s).(this is also the real reason that Itachi let Sasuke live)

The only know user of this "eternal Mangekyo Sharingan" is Madara Uchiha/Tobi.