[An AU where Sunset Shimmer never went through the mirror. The mirror portal didn't exist, and neither did the humans.]
When Sunset woke up to find herself in some part of Equestria she's never been before, she had originally planned to drive all the ponies apart in this place and gain more power to show Celestia that she's strong enough to be an Alicorn. But she didn't need to; all the ponies were already fighting with one another.
EDIT: Sequel is up now!