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To be honest, I think if you were to do this, you would not have one story, but two. One, the story of the world up until the villain wins, and Two, the story after the world changing event. So, ba...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/48597 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
To be honest, I think if you were to do this, you would not have one story, but **two**. One, the story of the world up until the villain wins, and Two, the story after the world changing event. So, basically, I would choose one and write that, and only that. Keep the other for a prequel/sequel. I honestly don't think you could write both arcs in one story without seriously alienating the reader (obviously not _everyone_, but I do think it would be the majority). I certainly wouldn't be interested in continuing to read something I'd invested my time in only to find out _poof_ the entire world and characters you came to love are no more! Let's start from square one. Yeah, no thank you. Readers will feel cheated at the very least, and there's a big chance that they'll not only put your book down out of frustration, but also have lost enough trust in you to not be able to trust any of your other books. But you're not asking whether people would want to read that kind of story, so much as you want to know how much you can get away with 'ruining' the world anyway, right? Personally, I would cut the 'normal world', the first 1/3 of the story, completely. Then, instead of showing Normal World -\> World Changing Event happens -\> Here's a Brand new World!, I would _start_ with the New World, and hint throughout the story that this **isn't what the world is meant to be** before finally revealing it to the reader, and maybe the characters, too. That, if done well, could make for an interesting psychological piece, with characters and situations that just don't quite seem to fit (maybe some characters were more resistant to the Change than others and have a sense kind of like _deja vu_ where 'This town doesn't feel right... Was it always so dark?' or 'Did this shop keeper always have such a nasty personality? What's happened to the caramel buns they used to sell!') but no one can figure out why, until it's revealed that this is a world the villain recreated to mirror his own perfect world. This way, you can still have the shock of the world and characters the reader invests themselves in having been dramatically altered (in the past), but it's delivered in a different way. While this may not be the kind of suggestion you would like, I, personally, think that it is all but impossible to simply write one story for 1/3 of a book before tearing it away from the reader and throwing one they very possibly don't give a fig about at their feet, and expecting them to be _happy_ with the shock factor. You are much more likely to entertain, but still surprise and excite, the reader by weaving the idea of a Changed World into the story in a more subtle and tactful way.