The legends foretold of six ponies who could stop her once a thousand years was up, but she escaped her lunar prison before her sentence ended, and those ponies were naught but a thought at that time.
Nightmare Moon had her revenge, and now ponies live in a world where no light shines. Nightmare didn't believe that was enough punishment for their sins, so she conspires a compromise: every year, each District will surrender a stallion and a mare to compete in the dreaded Equestria Games.
Rarity is a mare with an eye for fashion who's seen too much. Above all, she wants to protect her little sister from knowing and seeing the kind of horrors she has.