From Dust Till Dawn

Dust. It is the great tool of human and faunas kind, it was there weapon and defense, it was there finest tool and there end, for you see a double edged sword can cut from both sides. When jacques Schnee with the help of the Atlas military produced massive dust missiles that had automated assembly untill they where hacked turning them against those who made them causing the downfall of humanity. Thankfully human and faunas kind didn't go extinct they managed to survive inside massive underground tunnels but sadly the surface was not inhabitable for the amounts of dust used in the missiles caused deadly environmental hazards. After the years of living in the tunnels they started running low on supplies so they decided to send people out with semblances that negated the environmental hazards. Those who went to the surface are know as Walkers.