Transmigrated as The Forgotten Extra

Hundreds of years ago, a colossal comet struck the southern continent and spewed out a deadly virus that brought the world to the brink of destruction. The surviving humans sought refuge behind the walls or a steel fortress of the remaining civilizations. However, in the midst of a world decaying with metal and blood, a mysterious boy suddenly wakes up from his slumber. His name is Cain, an ordinary boy trapped in a reality that is not his own. Under a perpetual neon sky, Cain must struggle to survive, facing a new world filled with cruel fate and the horrors of terror that lurk beyond the iron walls. Then, when despair almost consumed him, a system window appeared before him. [Null Sequence requesting your permission to install!] A whisper of emptiness offered him salvation, which demanded a price from him. — Description from novelcodex.org