From the introduction:
Agartha is an advanced society built on technomagic - the fusion of magic and machinery. The whole world is covered by cityscape, housing tens of billions of human beings in conditions ranging from squalid subterranean slums to palatial towers of glass, stone, and steel. Even the oceans have largely been covered over, and the very weather has been tamed.
This city is rarely at peace. Dozens of factions vie for control over the world-city. Armed with the best in technomagical weaponry, from firearms to walking combat vehicles to great dreadnought airships, the factions wage long wars of conquest to expand their territory.
The factions only ceased fighting once, when the fiends of the Plane of Inferno invaded Agartha, seeking to enslave human ingenuity for their own ends. The city, faced with a seemingly endless horde of powerful immortal foes, created the Agarthan Directorate, an alliance tasked with driving the fiends from the city.
After almost a year of nonstop fighting, the Directorate managed to close the fiends’ portals, ending what has since become known as the Invasion War. But that was five years ago. The uneasy truce of the war has long since ended, and the factions of the city once again squabble amongst themselves. The Directorate’s power has waned, leaving the old factions to their own devices. The largest battlefield of the war has become the Scar, a blasted wasteland populated only by the most stubborn folk bent on reclaiming their lost homes and rebuilding the region.