Worldbuilding Challenge Day 5 - World Altering Event

What world event significantly altered the course of history?

2,000 years ago the great City of Eodia came to an end in one cataclysmic day, a disaster that would forever be named The Ending. No one knows the cause, but the histories remember a blinding flash of blue light coming from the very center of the Eodia. Then came from the city the Swirling Maelstrom, a electromagnetic storm that spread out from the Eodia like a ripple from a rock thrown in a pond. Anyone that was caught it it’s path was either killed outright or horribly mutated by it’s toxic energy. 

Yet through all the chaos, there was a ancient failsafe that activated as the toxic storm swirled out towards the Eight Cities. Through some form of Scyon power, each one of the Eight Cities was lifted into the air and shielded from the toxic energy. Floating above the storm, these 8 cities were the last bastions of civilization in Eodia. The energy from the Swirling Maelstrom eventually receded, and the Void Sea was the only thing left of the surface of the world. Although every now and then bits of the old world are spit up out of the Void to regain their place on the surface, as if out of the belly of some dark monstrosity.

For many years there was limited communication between the city-states. Messages were sent back and forth between the city-states through the use of birds, which could be unreliable. Thus each city-state became its own microcosm of ideals and values.