The world-ending calamity was, in fact, the introduction of magic. When it came during the latter half of the Industrial Revolution on this plane, not everyone that gained magical power was kind, or good. The First Global war was fought between vying factions of magic-users, with one side seeking to coexist with ‘Normals’ and the other side seeking to subjugate them as slaves. The old governments and much of the world’s technological marvels were destroyed, or lost. Infastructure and rule of law broke down.
When the campaign starts, one hundred years after the war, things have started to settle, and people are starting to crawl out of the darkness of that centuries-long war. Atrocities were committed on both sides, and huge swaths of the planet are now dead-zones, where very little can grow and dark and evil abominations stalk the land. Even in the places that still resemble the old cities, life is dangerous. The factions that went to war with each other no longer exist, but their attitudes and some of their members do.