War is the norm, rather than the exception, in and around the Wastelands. Usually the punchups between the governments of the coastal nations were small, not unlike the minor spats between our European nations. But sometimes, things got out of hand. Superweapons powered by magic and mad science had long existed, but their use was limited in nature, reserved more for threatening and posturing because of their terrifying potential. With the war in which that changed, everything ended in the Calamity.
It was a war unlike any other, a total war. More than just two or three nations at war, nearly a full quarter of the world's countries chose sides, and gave it everything they had. No weapon was too terrible, only a few were ill suited to some targets.
Hex carronades fired cursed shells, filled with the devil's breath. Wherever they hit, an evil mist filled with monsters blighted the land. A single shell was enough to turn a city to a ghost town; thousands of them were fired.
Magical backlash bombs destabilized the ambient mana, igniting manastorms. The already volatile magic of the wastelands went berserk, unleashing the fury of reality's bounds itself upon the world.
Chemical weapons of all sorts spread across the lands, from choking toxins that ate flesh from bone, to mutating agents that warped minds and turned plant and animal alike into ravenous horrors.
Nuclear weapons in all their various forms even made a few appearances, and a few more inventive and unusual incarnations charred the world to boot.
The tunnels, caves, and fortresses of the underdwelling races were hit with boring rockets, filled with a plethora of nightmarish payloads. Hex shells, backlash bombs, chemical weapons, nukes, the lot, focused into the protected cities, turning them into sealed tombs. Without the option of dissipation to the atmosphere, many will never be habitable again without dedicated purification. Those who survived were forced to the surface, which was by no means untouched.
Surface cities were ravished by the horrors, driving survivors underground, into the metro systems and bunkers. Rudimentary subterranean cities were improvised together, the inhabitants protected from the horrors by thick concrete and determined fighters.
And so, this transition of the old world to the new came to be known as The Calamity.
In the aftermath, the region's mana was reduced to negligible levels, granting protection from some of the warping effects of uncontrolled magic, but limiting the use of spells to ease the burden of survival. The ravaged ecosystem adapted, and those races that remained unwarped by the Calamity must maintain a constant vigil against the monstrosities that walk the land.
It has been 40 years since the world ended as it was known. Since then, things have settled down. The surface can be traversed, with enough protective equipment. In most places, a gas mask and a layer of tough clothing will suffice. The militaries of the city stations and station states have begun to reclaim the areas of surface around them, and underground frontier towns have been carved out. Old world equipment has been reclaimed where possible, and older designs modified and adapted for creation from the limited materials salvageable. Young recruits hold the lines against marauding animals, and brave adventurers range far from home in search of surviving treasures. All the while, new abilities have begun to manifest , and magic has begun to return, at least a little. Many of the mutations are helpful for survival.
Life isn't easy, but it isn't that bad.