demon apocalypse

story: it was just a normal day until the sky turned red and a massive creepy gate came out of the ground, then it opened all sorts of demons came rushing out killing every human in sight, not long after the army shows up and starts fighting back against the demons. the humans population drop from 7 billion to 15000, the survivors called that day the start of the demon apocalypse. you are military soldier looking around the ruin city looking for any supplies.