Older than Celestia herself, the Elements of Harmony were once in the care of the five most powerful beings known. This is the legacy of what they did to prepare the future to how it is now... 5000 years ago.
After Celestia humiliates her at a dinner, Luna storms off and ends up snooping around in her sister's room. There, she stumbles upon the journal Celestia kept immediately after the banishment of Nightmare Moon.
One morning while raising the sun and lowering the moon, the princesses sense a disturbance that leaves them unsettled. On the same day, a traveler arrives in Ponyville.