“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, Earth, was a talented man, and everyone expected him to succeed his father as a warrior and defend his country, but Earth was always wary of his father’s title.
One day, Earth 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.”
Earth 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’s parents, the heroes of the past, the first love, the princess childhood friend, the geniuses of the era, and the world will tremble.
Chapter 29 - 
Chapter 402: Exchange Conditions
Chapter 131: High School Student Yoo Ha-yeon (1)
v2 Chapter 15 clarify
