A Breakthrough Came Out by Forbidden Master and Disciple.

The hero's son and the ghost of the Demon King are the master and pupil!? Eirth, the son of the hero who defeated the Great Demon King, is troubled. Always expected to achieve great things, but unable to meet those expectations. No matter the effort he puts in, and the results he obtains, it's never enough for them. "I'm a disappointment compared to my father" or "Don't you know you're the son of the hero!!" Living every day like this, Eirth meets his destiny in the room containing the sword of his father, the hero. "Interesting. I will train you. I have only free time now." Eirth receives special personal training from the Demon King just to get back at his parents and the world. In the coming days, his parents, the heroes of the past, his first love, the princess childhood friend, the geniuses of the era, and the world will tremble before him. (Source: MU)