When Celestia and Luna were corrupted by their immense power, Cadance summoned the magic of the Elements of Harmony and overthrew Equestria's wicked princesses. She then claimed the throne for herself,
But the Elements were not meant to be wielded by Cadance. When a streak of lightning causes six ponies to simultaneously gain their cutie marks, Cadance knows these six are the true bearers. She takes the eldest, Amethyst Star, as her student, and moves the rest to Ponyville.
Just remember, my little ponies, love conquers all.