Eight years ago, an unknown catastrophe called (“Demonization”) struck the world. It is a calamity that seals off parts of reality and overwrites them with a nightmare realm—if you cannot survive, only a cruel death awaits. In this changed world, Yūgo Fushimi lives a quiet, withdrawn life, limiting his outings to commuting between home and work. Gentle, timid, and overly sensitive to others’ pain, he is always sleep‑deprived and often late, leading his coworkers to mock him as a “clumsy do-gooder” and a “shut‑in introvert.” One day, he is forcibly dragged out to a drinking party—and there, he witnesses the horror of Demonization firsthand. The city is trampled, people scream and cry, his coworkers collapse one after another, and grotesque enemies flood the area as it transforms into a demon realm where survival seems impossible. At that moment, in the midst of despair, Yūgo alone grips his weapon as if remembering something long forgotten—hinting that he may be a “returner,” someone with a hidden past connection to this nightmare and the power to fight back.

The Demonic Cult Leader, Seok Woncho, was a monster who even surpassed the Blood Cult Leader.Do you know who killed him?"Ha Mu-baek.""But why are you bringing that up?""Because he's coming to our Mengryong Academy as an instructor."Da** it. We're screwed!Ha Mu-baek, the greatest martial artist who crushed both the Demonic Cult and the Blood Cult,is now assigned as an instructor at a martial arts training academy due to a political power struggle.Determined to live quietly while keeping his identity hidden,he soon finds himself solving the problems of cadets with heart-wrenching backstories,disciplining the trashy human scum at the academy, and inexplicably becoming busier by the dayWelcome! It's your first time meeting a genius lunatic instructor, isn't it?!