Cassius was a demon king who was killed by a hero one day.
But for some reason she still remains in the world as a spirit. She witnessed everything the hero did after defeating the Demon King. In the end, Cassius learned that he was the villain BOSS in the novel "Become a Brave" and was reborn twenty years before he was killed.
After rebirth, Cassius decided that she would take revenge on the brave man, not only for herself, but also for everyone who had been let down by the brave man.
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Tifa is the Elf Queen and the heroine in "Become a Brave". He was reborn after being killed by a hero.
Double birth, the sultry devil is attacked by the wife queen
The main perspective of the Demon King will also be mixed with the perspective of the Elf Queen.