Sunset Shimmer never defeated a powerful alicorn to conquer the world and become a princess. She's just a student at Canterlot High School.
But sometimes, she has unpleasant dreams she can't explain.
This story might be an entry in Imposing Sovereigns IV, using the prompt Sunset Shimmer / Persistence. ('Harmony' and 'Strength' would also be possibilities.)
Prereaders: Punished Bean, sirenc0re. As usual, responsibility for the story's faults remains my own, as do my limitations and shortcomings in trying to apply the prereaders' feedback and advice. 
Chapters (1)
When a mare has power she doesn't want thrust upon her, sometimes her mind finds ways to vent its frustrations over such an event...
This makes me wonder which one is the dream.
This is certainly an Inception-like ambiguity. Well done.
Ah, school dreams. Goodness knows I still have them sometimes. You spend a good chunk of your formative years in the same building and it’s going to leave a lasting impact.
Fascinating bit of slow-boil horror. I wasn’t sure where this was going, but the reveal and its implications… Yeah, very good stuff. Thank you for it, and best of luck in the judging.