A Breakthrough Brought by Forbidden Master and DiscipleKindan Shitei de Breakthrough ~Boy Meets Satan~; 禁断師弟でブレイクスルー　～ボーイ・ミーツ・サタン～; 禁断師弟でブレイクスルー～勇者の息子が魔王の弟子で何が悪い～; Kindan Shitei de Breakthrough ~Yuusha no Musuko ga Maou no Deshi de Nani ga Warui~; A Breakthrough Came Out by Forbidden Master and Disciple.; A Breakthrough Brought Out By Forbidden Master And Disciple ~Boy Meets Satan~; A Breakthrough Brought Out By Forbidden Master And Disciple ~What's Wrong With The Hero's Son Being The Demon King's Disciple?~; Breakthrough with the Forbidden Master: So What If the Hero's Son Is the Demon King's Disciple; Kahramanın Oğlu ve İblis Kral Öğretmeni; Eirth, apprenti du roi démon

“Compared to our time, the current generation just aren’t as reliable” they said, “Still, isn’t he the son of the Hero?” How annoying. It’s been over ten years since the battle between mankind and the demons ended. The children of the heroes who saved the world are all grown up. One of them, Eirth, was a talented man, and everyone expected him to succeed his father as a warrior and defend his country, but Eirth was always wary of his father’s title. One day, Eirth is shocked to find his father’s sword, which was used to overthrow the Demon King, sealed in his family’s mansion. Because trapped within was the ghost of the Great Demon King who was unable to pass on and continued to roam the world. For some reason, only he could see the spirit of the Demon King, and the Demon King possessed him through the sword. Every day, forced to coexist with the spirit of the Demon King, they live a strange shared life, when the Demon King whispers to Earth on a whim. “I have a grudge against your father. If you want to get back at your father and the world, I’ll train you. I’m free.” Eirth receives special personal guidance from the Demon King just to get back at his parents and the world. Then, in the coming days. The hero parents, the heroes of the past, the first love, the princess childhood friend, the geniuses of the era, and the world will tremble.