The Regressor Can Make Them All

Lee Se‑hoon, the blacksmith of humanity's final resistance squad, fought demons to the bitter end, only for the battle to end in mutual annihilation and the complete destruction of the world. Faced with that futile conclusion, he takes his own life—"...What the hell is this?"—only to find himself back in his candidate days at Babel, the hero‑training institution. Given a second chance, Se‑hoon resolves to use anything and everything as material if it will lead to victory. Anything.

(Source: Kakao Entertainment, translated)