When Celestia and Luna tell Twilight that being an alicorn comes with challenges and trials, and that she needs to control herself and her magic, they refer her to an old book on meditation. Twilight is skeptical at first, but she reads it anyway.
When she gets into it she finds that the book is not filled with sensible, helpful tips on meditation at all, but the message inside gets into her head. Eventually she starts having visions, and one vision in particular will not leave her mind until she brings it into the real world. The consequences change her views of Ponyville - and her duty to it - completely.