The Mage and the Filly Fair

A stranger that nopony can remember brings a young Sunset Shimmer three gifts. They turn out to be curses.

Before Sunset Shimmer tried to take over a high school, long before she fled through the mirror portal, in the days before she was even the personal student of Princess Celestia, the young filly met Canterlot's most elusive stranger. He gave her a secret. And then a gift. And finally, a revelation.

In the end, he really only brought Sunset a curse.