I remember a story from a long while ago.

It was a medium to long fic.

Twilight somehow went to an alternate universe where Cadance ruled, except she was corrupt, like Daybeaker or Nightmare Moon. I can't remember her evil name, though.

Twilight joined with a resistance group to defeat her.

Anyone remember this?

Anyone out there, for that matter? ;)

Well, I'm curious to see how the author handled it.

On a more general note, I see here are some ideas about Cadance as villain: https://www.reddit.com/r/mylittlepony/comments/xqm5xt/daybreaker_and_nightmare_moon_what_should/

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Well, I found it, finally. Downloaded the FIMFictionArchive, converted the entire thing to plain text, and then spent about two days using AgentRansack to search for random collections of words that seemed to feel right.

It's not quite as I remembered it... I remembered it being largely Twilight-centric, and it wasn't. There's an A-plot and a B-plot, and Twilight is the A-plot, but there's still a lot of other stuff going on. It's also not a stand-alone work, it's one installment of a multi-work series. And a decidedly NSFW one, if that matters. It's Who Rules?: Shattered Hearts, the fifth installment of the Who Rules series.

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Congratulations on finding it! Too bad I haven't been able to easily find the story with that title using fimfiction.net story search.