The King of the Battlefield

Humanity was on the brink of destruction. Muyoung, an assassin from the Forest of Death, fell to his knees. There were corpses all around him. For years, he had worked for the Forest of Death and had finally been able to eradicate them. Now, he, himself, was about to die. He closed his eyes as he felt his heart stop beating. Suddenly, Muyoung opened his eyes. By some miracle, he was still alive. He then realized that he had actually gone back 40 years in time. Join him as he decides to fix humanity’s past mistakes and fight against the 72 demons of the Underworld. — Description from novelcodex.org