This story is a sequel to My Little Pony: Nine Elements Book 3
The story continues with season 4, where Sunset Shimmer has returned to Equestria as opposed to staying in the human world.
This retelling is partially inspired by SuperPinkBrony12's MLP (What If?) series. (Start here if you're interested.)
My Little Pony (C) Hasbro.
NOTE: The anthro tag applies because Sunset made a device that allows ponies to become anthro, which is revealed later.