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

Synopsis:

Is the hero's son and the ghost of the Demon King a master and disciple? Eirth, the son of the hero who vanquished the Great Demon King, is struggling with pressure. Constantly expected to live up to legendary achievements, he finds himself falling short. No matter how hard he tries or what results he delivers, it's never enough. "I'm a failure compared to my father," or "You're the son of a hero, don't you know?" With this weight on his shoulders every day, Eirth's fate takes a turn when he enters the room that holds his father’s sword. "Interesting. I'll train you. I have nothing but free time now," says the ghost of the Demon King. Eirth agrees to receive personal training from the Demon King, driven by the desire to prove himself and get back at his parents and the world. In the future, his parents, the legendary heroes, his childhood love—the princess, the geniuses of the age, and the world itself will tremble before him.