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.