Kenja no Tsurugi

A man who died in an accident is reborn in a new world as an infant, beginning a second life. He discovers this world is based on a game he once played, and that he was reborn as a character who dies early in the game's story. To avoid this fate, he uses his knowledge of the game to improve his abilities. He quickly gains exceptional power and skill, then sets out toward the continent targeted for conquest by the Demon Lord.