Chaos reigns sketch
More sketches. Harbingers burst through a vaulted tunnel.
Styx is an iceberg. The city is on top, the rest of its massive architecture digs below the earth, almost as an entirely different city cast in a constant air of nightlife with food and drink marketplaces; and corporate and dark gatherings. When the Harbingers invade, this goes too.
There are many networks of vaults that lead from the tendrils of the underground out to the desert as a means of emergency evacuation. The apocalypse happened once before, but no amount of escape routes can prevent a sudden, chaotic clog of hellhounds.