There is an unknown fact that Luna had been hiding about herself : She has been killing ponies in their sleep. What could the reasons be that drives the Princess of the Moon to do this? What will happen when her dark secret comes to light?
Luna has fallen into a deep sleep and cannot be awoken. Meanwhile, the night sky dims more and more with every passing moment. The Mane 6 are sent underneath Canterlot for a potential solution while Celestia works on her end.
Where was Luna during the attack on Canterlot? Why did she not aid her sister in protecting the city? Perhaps she was protecting other ponies... or perhaps something more sinister occurred.
Where does evil lie? In the heart? The mind? Or the soul? The result is the same. Such darkness is all consuming. Why not see for yourself, of what lies in Luna's mind.