This story is a prequel to The Amazing Sunset Shimmer
https://www.fimfiction.net/story/467263/the-amazing-sunset-shimmer

Because she sealed a pact with Mephisto Pheles, a famous stuntgirl becomes, by night, the armed arm of the prince of evil whose disastrous hegemony is coveted by his own son.

Death-dodger by day, biker vigilante after the sunset,
Spitfire will have to fight to save the ones she cares about.

This story takes place in the EGFU: Equestria Girls Fiction Universe