@Sliwith ~cast a high level spell to shiny a bit~ "i am the reincarnation of the thunder sage, and i have came from the heavens to restore the honor and glory of our family, i am using this boy body as my avatar any sins and shameful behavior will be punished, i have no problems on killing all impure members of this family and starting over" people are not that smart, once we believed a story about a dude that turned water on wine and walked on water, that was enough to make part of our civilization believe that he was son of god or something like that, there was consequences but according to the tale he chose to suffer the consequences, the main character on this manga have actual power and his goal on my statement would be far easier that the other example i made.
- ancient lost magic -check
- full of power - check
- commanding voice -check
To me that would be enough for him to restore himself as the head of the family, but he does not need that, with the power he have its easier to accumulate power, respect and achievements on himself, them create a branch of his family starting with him his mother and future friends/lovers would make a new family, and that sounds more like something this main character would do.
Worrying about executions and other stuff like that on your comment when he is that powerful is funny, and he have the experience of founding a great family name, protecting his mother is just a small easy problem to solve.