After you beat the game you must travel through Konoha heading towards the Hokage building (constantly going as straight and up ass possible. Just before going up the huge flight of stairs (Lee, Tenten, Neji, and Gai chill there) to the left is a frog with a purple mission scroll over him. He gives you a quest to collect chakra fragments and after collecting so many of them (about the fifth time you are sent back out to collect them...frog statue should be red), he will send you to the East Lightning Coast to get a 'special fragment'
Then go to the East lightning coast and make your way to the Final Battle arena stage...open field with some fences. When you are there it will play battle music and at the center of the arena will be a black cloud like the ones that imposter Zetsu have, but a purple gem sits on the bottom.
Walk up to it and you will go through 2 missions as the Eternal Mangekyou Sasuke!
I kinda gave a roughly detailed exolaination...send me a PM if you have any questions! I'll switch to my laptop for a more detailed explaination if you so desire!
EDIT: Forgot to mention completing this gets you EMS Sasuke....and there is only one fragment
