In another universe, far from the one we know (but not too far), a butterfly flapped its wings. A baby was born, as we well know. But rather than being white, with a mane formed from a prism of color, this filly was blue, close to pitch, with a softer blue mane. Her parents lovingly named her Luna. Not too long after, they had another baby. White, this time. I'm sure you know what to imagine. They named this filly Celestia; thus, our two royal sisters were born.
And, due to this one small change in what was now ancient history, we must begin anew; hear the story from the beginning. A familiar story, and yet so very, very different.

~

Over a thousand years later, a young unicorn mare named Sunset Shimmer studies under the ruling princess of Equestria, Luna. With her sharp mind and spiky exterior, she's more than happy to spend her days alone, dedicated to her magical studies. She's made great strides with no one but her teacher and dutiful assistant, but when Equestria is faced with a forgotten villain of legend and she's the only one left who knows how to defeat her, "going it alone" may not be an option any longer.

CHANGED SOLAR FLARE TO DAYBREAKER FOR CLARITY'S SAKE

I've also posted this on Ao3!
Cover art is mine :)
For those of you who are as visual as me and want to see the references I drew up for myself, you can find them here!