The story begins with the mysterious black fog that started emanating from an underwater volcano one day. Neither the raging waves nor the howling winds could stop its spread. However, at first, no one paid much attention to it and simply regarded it as a natural phenomenon. It wasn't until the first person was swallowed and killed by the black fog that people began to panic. The black fog slowly crept towards the city, and after exhausting all options, people turned to building underground cities and living below ground. However, everyone knew that the black fog would eventually permeate the underground city. This was the beginning of the Era of the Black Fog.
