There is an unspoken rule between the two ruling Princesses; the Sun and Moon must not be in the sky at the same time. However, this rule had just existed out of common sense that they would simply take turns raising their respective celestial bodies.

One day the artistic Luna gets the idea of being able to show her work once a year during the day with something called a Solar Eclipse. Celestia hesitates but agrees to give it a trial run before making it an annual occurence. It doesn't go quite as planned and the pony princesses now find themselves with a creature possessing what could be the most terrifying ability for a world full of magic