There are two immortal characters. One is 'immortal' in the sense that he does die like a normal human but is always reborn with all of his memories intact, and so remembers all of his lives back to the beginning of time, and has a lot of suffering due to the fact that everyone he's love in that time has died and he can't get close to anyone because he knows that in the end they'll die and he'll remember and only suffer from losing them.
So, that guy does 2 crazy things: (1) He uses some crazy part-science, part-magic/religious method he has developed over several lifetimes of study, and makes the body of his best friend in the current life immortal. (2) He decides that the next time he's reborn, he will destroy the world so that there will be no woman left to give birth to him, so he can finally stop being reborn. He tells his best friend something like "You will live forever, so you will understand my pain. Every time I'm reborn, you must find me wherever I am and you must kill me, or I will destroy the world. When you finally suffer as much as I have you will come to understand my way of thinking, and in that lifetime we'll destroy the world together." However the friend who was made immortal (the main character of the series, btw) is an idealist and wants to save both the world and his friend.