For all that we know, Luna acted of her own accord that night.
Fed up with being just the night, she tried to overthrow Celestia.
But what if that was wrong? What if something darker had happened, a force that we don't know about, had came into play?
It's true. A nightmare creature, Lunar Essence, had sent itself to Luna during an eclipse on that night, and took her body for himself. But Celestia was stronger, she was able to overpower the being, and banish it to the moon. The ponies of Equestria could not know of this being, so she made it appear as if Luna had been banished. When it made a second attempt on her, the Elements of harmony had destroyed it for good.
Or so we thought.
Rather, it lives again on the desolate landscape, and has this time taken Celestia.
One set of Elements won't be enough to stop this newly reborn monstrosity, so Twilight makes a drastic decision. Joining her to stop the host of horrors, she summons two more Elements. The Human Elements, and the original Elements, but will this even be enough?