The worst possible thing I can imagine?

The moon falling from the sky and crashing into the land.  It crushes two cities at once.  The world shakes and the tides stop, and nothing is quite the same again.

Have I thought about this before?  Yes.  When Sage was much younger, their teacher asked them to make a model of our continent, so we tried to make a sort of sculpture out of cake.  It had little cities of frosting on one side and a forest on the other, with clouds on top of the forest, and some wooden sticks through those layers to support the moon hanging over everything.  But the sticks broke, and the moon fell.

Sage had nightmares that week.