The catastrophe didn't only affect humans, either. It affected the whole natural world. Objects spontaneously combusted, freak storm appeared in seconds from nowhere, the earth shook, animals died on their feet, forests withered. Stranger things were reported as well, the stuff of fairy stories: you could go for an hour's walk and return days later, or walk through a door in your own house and emerge into a room in someone else's. There were tales of travelers meeting themselves on the road.
The catastrophe was relatively brief - a matter of weeks - but that short time changed everything
What happened was this: spirits - the powerful, incomprehensible, non-material, and utterly alien beings summoned by sorcerers to work magic - suddenly rushed into the world, unsummoned and utterly uncontrolled. They didn't have an agenda; they came unwillingly, and acted more from fear and confusion than anything else. Mostly, they didn't even mean to do harm. Their world, their experience, is vastly different from ours. They didn't know how to interact with this world, or even how to understand their perceptions of it clearly. They were, effectively, bulls in a china shop, smashing everything they touched - but invisible bulls not strictly bounded by natural laws. At the time, everything was all the more frightening because the average citizen knew nothing about spirits and had no idea what was happening, but practicioners and scholars of magic figured it out, and afterward the story spread.
The catastrophe ended when the spirits found a way back to/were pulled back into their own world. It's not clear what actually happened. But, and this is the key point, everyone thinks they know what happened. Namely, the the Hero, or whatever, a messiah figure, went on a Quest with his Plucky Friends, defeated the Big Bad and Saved The Day. And, someday, when he or she is needed again, the Hero will Come Back and Save Us All.
But the thing is that there never was a hero: the story was spread by priests, crafty priests who perceived both danger and opportunity in the end of the catastrophe. Effectively, they swept in and took credit for the end of the catastrophe, that the Hero, Messiah, whatever, with the help and guidance of their god and their order, had saved everything. They acted quickly and decisively and weren't shy about co-opting existing stories and rumors, and their version became the "accepted" version. Then they wrote a definitive text and set about revising history as aggressively as possible.
Their motivations were two-fold: on the one hand, this was a complete get for their order. Their god became all-important, their earthly prestige skyrocketed. On the other, the story succeeded precisely because it gave people exactly what they needed in the wake of the catastrophe. People were afraid; awful things had happened for no reason at all, and there was nothing they could do about it. The lie explained everything in a neat bow: evil forces had brought devastation, but the forces of good had won the day. Evil would be back, but not right now, and good would defeat it again as needed.
Ultimately, the lie did a lot of good. It arguably saved civilization, maintaining the old order in the wake of chaos - although that included old injustices along with old securities. But the hope that it brought, though badly needed, was accompanied by complacency. Because "the Hero" had saved the day, and would again, everyone has spent the several hundred years since the last catastrophe not worrying about what happened, and why, and how to stop it from ever happening again.
And now, it's about to.