Densetsu no Ore

Ozma is the fomer court magician of the defeated Kingdom of Edward, now living in the slums to hide from the Empire, until he revealed himself to save Felia, a young girl about to be sold to a brothel. He forced Felia to report his whereabouts to the Empire and take the bounty on his head, even though that meant he was executed. However, he was reincarnated into the same world 300 years later. There, his former self, who was supposed to have died with the execution, has instead become an important name in history, with many great accomplishments that he has no recollection of. Now he is praised as if he were a god, a legendary hero!

(Source: Hifumishobo, translated)