The princesses gave their magic to Twilight to prevent Tirek from stealing it. As a result, they end up stuck in Tartarus. The fact that things manage to go downhill here says a lot about the wonders of family.
Canterlot has fallen at last, and six heroes go to face the tyrant of Equestria: Princess Twilight Sparkle. What they find, however, is not at all what they expected. — A reaction to Tyrant Sparkle fics.