Panacea Bo, ancient unicorn healer. When a long lost relic of hers is uncovered, it's research falls upon Twilight. Said to cure any disease, Twilight and Pinkie Pie test it's power!
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.