Zamasu and Goku black may be a different body but they are one in the same Zamasu of another time line stole to powerful mortal known as goku as Sayian with extraordinary every time he edges death he gets an insane boost in power making Black the power house of the duo and zamasu from his time line wished for immortality making him unable to die healing any wound no matter how severe. they are on a quest to extinguish all mortals as they believe they are the incrimination of the universes