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.
The adventures of the Mane 6 as the Power Ponies as they defend Maretropolis from dastardly evil-doers, including the Mane-iac, the Equalizer, the Phoenix, and the God of Chaos himself.