A "Dark Heal" — a mysterious power that harms the holy and heals the demonic. Kiwi Araya, a human born with magical power, lived a peaceful life healing pets and harmless monsters. However, when war breaks out between humans and demons, he loses his freedom under human rule. In response, Kiwi promptly defects to the Demon Realm. Upon beginning work in the Demon King’s Castle, he discovers a literal paradise: a place overflowing with research subjects and untapped business opportunities. "Let’s begin with the diagnosis. Why does the lewd crest glow? I want to know the conditions and the reason it shines… Does it glow? Or not!? Come on—tell me!" This is the story of a genius who seeks nothing but knowledge and profit, yet through his unexpected actions, rises to fame and eventually becomes a legend.