What's the worst thing you've done to a world?

i ended it, simply by having a particular character exist

and no practical way to get rid of her, even when the world ends

before that several very large chunks of history were wiped out thanks to the aforementioned character, leaving the abominations that govern the world even more confused then they already were.

I've got a lot of works in the planning stages (most set in their own universe), and I wanna cap them all off with a Grand Finale Crisis Crossover, where a villain gets ahold of the most powerful MacGuffin in The Multiverse.

So, the worst thing I'll be doing to the multiverse is probably have the villain unintentionally screw up the metacosmos, wiping out millions of universes, and leaving the remaining ones in disarray.

Still trying to figure out how to make it a comedy.

I made a country revert back to the time before indoor toilets, electricity, running water, and the Internet. The characters were very used to the creature comforts of the 2000s, so this was a shock to their systems.

But then it went back to normal within a year because I got tired of writing the "after the end" type scenario. I don't know if that plotline will make it to the final draft.

Half of my world is so heavily soaked in radiation that it is incapable of sustaining any life and the other half is a barren wasteland that is not as radiated as the other half. On the better half is one supercity/nation, in which the remainder of humanity remains, not counting one or two bodies preserved in a far-off location.

No one in the city knows that the outside world is dead. They think that their city-nation is the entire world. The society of the city-nation is a horrible, unfair thing obsessed with appearance and holds a horrible poor-rich system. The villain, who caused the nuclear apocalypse, is trying to finish the job, and is an AI that is very close to achieving the necessary power to destroy the evil remnants of what is left of civilization.