The Indomitable Martial King

Repenhardt, the most powerful 10th circle wizard on the continent, stood for the liberation of different races treated as s*aves across the land.
“Where is the flaw in such a noble thought?”
Thus, he was dubbed the Demon King by humanity.
“However, following the distorted common sense of a twisted society, knowing it’s wrong but acknowledging it because everyone else does, rather than showing humility…”
“…I would rather become arrogant.”
Defeated in humanity’s final battle, he used a time regression spell to return to the past…
“Who are you?”
He awakens reincarnated in the body of the continent’s strongest physical warrior, Teslon of the Unbreakable Gym, known for his headbutting prowess! What is the fate of the former Demon King and 10th circle wizard?