I Am Studying in the Barrier of Life and Death

People who wake up are in a mirror world. Outside of here, civilization is in danger. The inner world is obsessed with assimilating human beings. The ghost world is crazy about killing people. With the combined efforts of the three parties, humanity has declined. A full-scale invasion is inevitable. Where is the so-called way to survive? They shouted: Keep on going on the civilized journey! The person who wakes up is in a... Mirror world?