Re: IC - Pathfinder e6 - Post-apocalyptic fantasy
Sudden unexpected magical cataclysm? The last time I wanted one of those in a backstory I borrowed the comet from the SNES game 'The Illusion of Gaia'.
The comet came one night, lighting the sky with its corrupt radiance. The comet's light mutated and twisted life, shattering the old world in a night of madness and horror. Most of the comet-twisted lost their minds, and even some of those who retained sanity were deeply changed, some to the point of considering their former self dead and their current self a new being. Even centuries later, the world lived in fear that the comet may someday come again.
There's also the idea of a cosmological shift in magic, ruining the civilization that depended on it, and forcing the survivors to start studying magic from scratch.
Good luck with your game!