[Theory] The truth behind the Sharingan & Mangekyou Sharingan
Hello everyone. This thought came to my mind after Tobirama revealed that the Sharingan is activated when some kind of chakra is released into the Optic nerves. He also stated that the only reason for the Sharingan's activation is because the Uchiha had a great attachment to love, and if that one Uchiha individual loses it, a special kind of chakra is released towards their optic nerves, as stated above.
Now, in the case of the Mangekyou Sharingan, in my opinion, is that it's the same process as awakening the Sharingan, but you(the bearer of the eyes) has to kill their best/closest friend, in order to obtain those eyes. Yes, I believe another 'type' of chakra is released and sent to the Optic nerves for the activation.
BUT, I aswell as others may have noticed(pretty much entire Narutoverse that reads Manga) that Uchiha's who have the Mangekyou Sharingan, their eyesight deteriorates, and sending them into complete darkness.
I think this is because that they were being consumed too much by their ongoing hatred, and that their eyesight was the price.