The Eternal Penance is a technique that uses the old saying 'The eyes are the windows to the soul' and takes it to a whole new level. It destroys the mind by taking all of the pain that the person you use it on has caused others in his/her life, multiplies it by the number of eyes made for the Genjutsu, and turns it back on the person. You control the amount of time that all of this takes, so you can make them feel the pain in a sudden rush and totally obliterate their mind, or torture them over the course of years. Kind of like the Tsukuyomi's seventy-two hour thing.