Upgrade to Mangekyo Rinnegan

The Mangekyo Rinnegan is an upgraded version of the Rin'negan, you just have to see your two best friends die. You don't actually have to do it yourself. You they can copy jutsu like with the Sharingan, get 3 Mangekyo Sharingan attacks and the ninjutsu and taijutsu of the Hyuga clan.

Known Users
The only known user is Carmaru Rinnegan because he saw Roushi and Yugito Nii die at the hands of the Akatsuki.