What is the worst thing that could happen to your world, and how would it change your world?

So for my world building I've had the idea that a sort of 'Great Drying' where all the oceans dry up and turn into salt lakes happened 3,000 years ago or so. I'm thinking of a lot of species went extinct as a result and the climate changed a lot, and a lot of the technology of the time would have been rendered useless. This was a turning point for the world, after this point there were many empires, wars, and upheavals as the world adapted to the new world. <br><br>&#x200B;<br><br>This is a great backstory, and I feel like this would be a good point of divergence for my story, but I'd love some ideas on what other possibilities there are. What could possibly be the worst thing that could happen to your world, and how would that change it? I'm thinking massive war, alien invasion, huge volcanic eruption, massive disease outbreak, and so on.<br><br>&#x200B;<br><br>Also if you have ideas of how you'd personally react to such events, that would be cool too!