The plane known as “Armageddon” is an Earth Shard.
Once home to a technologically advanced (but magically primative) culture, Armageddon’s fate was sealed when it began to slide in to phase with the Abyss. Eventually, a planar eruption occured, linking one of Armageddon’s most populous cities with a deep layer of the Abyss. Through this portal, the Demon Overlord Cyphiax launched an invasion.
With no magical traditions to speak of, the defenders of the plane were largely overwhelmed. Their tanks and planes and bombs initially proved at least somewhat effective, but before long Cyphiax’s forces adapted. Incorporeal spirits, summoned creatures, and powerful spellcasting soon made short work of conventional military forces, and when Cyphiax unleashed his “Four Horsemen”, powerful demi-demons, the defenders were quickly overwhelmed. In a panic, the governments of the world attempted to destroy the points of entry for the demonic forces with nuclear weaponry.
While numerous “demon holes” were effectively buried underground by the explosions, the radiation had no effect at all on the demons themselves. At the same time, the explosions blackened the sky and irradiated the landscape. Rather than halting the demon advance, it only managed to destroy the planet’s biosphere, and scatter it’s people even further.
In the span of a generation, this once thriving world has been transformed in to an apocalyptic nightmare, the world called Armageddon. With the major governments and military forces of the world lying in ruins, Cyphiax has not seen fit to deploy a major demonic contingent to Armageddon in some time, likely utilizing the bulk of his forces on other battlegrounds throughout the multiverse. Instead, the Four Horseman and several demonic lieutenants oversee the continued subjugation and eradication of the local populace.
The people of Armageddon are typically hard bitten sorts, used to scavenging for supplies and fighting off demonic raiding parties. Save for a few isolated pockets of military resistance, the human forces are largely defeated, thedays of “total warfare” are mostly over. They mostly live in small communities and heavily defended outposts, struggling to survive in the new, harsh climate.
A few pockets of military resistance remain, most notably the settlement of New Ark, possibly the largest human outpost on Armageddon. The outpost is especially noteworthy given it’s proximity to the largest Hellhole in what was once New York City. New Ark’s continued survival is largely dependent on it’s sizable military force, made up of survivors of the Demon War and their descendants. In addition to the defensive forces for the settlement, New Ark is home to the Helldivers, an elite military unit.
Despite New Ark’s successes, the only reason it continues to thrive is because Ciphiax has not seen fit to reinforce his demonic minions on Armageddon to make a push to destroy it. They just don’t do enough damage to be worth the effort. Should he ever turn his full attention to the city, it would likely fall within hours.