In this AU, Twilight has the character journey that Starlight in the main continuity did.

When Twilight was a foal, she ended up getting her cutie mark earlier than other foals, including her best friend, Moondancer. Before she even had a chance to say goodbye, she was sent away to Canterlot because of her magical abilities and potential. However, being separated from her best friend and family, Twilight grew bitter and would eventually run away from the school, blaming it all on cutie marks and how her life went downhill all because she got her’s.

Soon, she started to force her philosophy onto other ponies and managed to create a spell that stripped them of their marks and talents, leaving only an “X” in it’s place. Twilight secretly kept her mark and all her magical abilities in secret until Starlight and her friends came across her village and exposed her, freeing the other ponies and bringing their marks back to them while Twilight escaped to fight another day.

When Twilight returned, she tried to do what she did again but to all of Equestria and was almost about to do it until Starlight managed to make her see reason and surrender after she looked through old canterlot records and learned about her past.

After this event, Twilight became a student to Starlight and has become a much better mare than she once was.