Kindan Shitei de Breakthrough: Yuusha no Musuko ga Maou no Deshi de Nani ga Warui
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～
A decade after the war between humanity and demons ended, the children of the heroes have grown into young adults. Earth, the son of a legendary warrior, was expected to follow in his father's footsteps, but he has always questioned his father's legacy. One day, he discovers his family’s ancestral sword, which was used to defeat the Demon King, sealed within their home. Inside is the spirit of the fallen Demon King, who cannot move on and now haunts Earth. Only he can see the spirit, and the Demon King begins to possess him through the sword. The two develop an unusual coexistence, and the Demon King offers to train Earth to seek revenge on his father and the world. As Earth accepts this guidance, those around him—his parents, former heroes, childhood friends, and other notable figures—will soon feel the consequences.