I had an idea for a story where Chrysalis returned earlier (Mid-season 4) and wins. The Mane Six and the Princesses (Except Cadance) are presumed dead and soon after all of Equestria falls under Changeling control. Here is the problem: Not only is victory boring but it turns out that most ponies don't really love it when someone kills their rulers and heroes. Oh not to mention the Paperwork that comes with ruling Equestria.....And Tirek just returned. Great just great. The story is called Bittersweet Victory.
Also looking for a Co-Writer

I think it would be allowed here
a similar story also in This Group

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It would fit if, at the end, Chrysalis becomes a hero to the ponies.

That is the end goal