Kindan Shitei de Breakthrough: Yuusha no Musuko ga Maou no Deshi de Nani ga Warui

More than a decade has passed since humanity triumphed over the demon race, and the children of the legendary heroes who saved the world have come of age. Among them is Earth, a young man burdened by immense expectations. Society constantly compares him to his father, assuming he will naturally inherit the Hero's mantle and defend the nation. However, Earth has always felt suffocated by the heavy shadow of his father's legacy and the endless whispers that he can never measure up. Everything changes when Earth discovers his father's legendary sword—the very blade used to slay the Demon King—sealed away within his family's estate. To his shock, the weapon harbors the lingering spirit of the Great Demon King himself, unable to pass on. For reasons unknown, only Earth can perceive this phantom, and the Demon King soon uses the sword to possess him, forcing the two into an uneasy coexistence. During their bizarre shared existence, the Demon King makes Earth an unexpected offer: fueled by a deep grudge against Earth's father, he proposes to personally train the boy if he wishes to exact revenge on both his parent and the world that shackles him. Accepting the forbidden tutelage, Earth begins receiving secret, specialized instruction from the greatest enemy mankind ever faced. Armed with the Demon King's power and knowledge, Earth prepares to shatter the expectations placed upon him. In the days to come, his parents, the veteran heroes of the past, his first love, his childhood friend the princess, and the era's greatest geniuses will all be left trembling before the unprecedented strength of the Hero's son.