After her feelings of jealousy for her sister boil over Princess Luna becomes Nightmare Moon and seeks to overthrow Celestia, only to be banished by the element of harmony. However the elements did not send her to the moon like they should have, instead she was sent to a far off planet known as Cybertron where she encounters it's rulers, the mighty Megatron and his Decepticon army. Now the two of them are plotting together and awaiting the day when the banishment is over and the portal to Equestria reopens.

Cover art, which also is what gave me the idea, belongs to IronRaptor. Check them out on DeviantArt