Disclaimer: I own absolutely NOTHING! Except for the Nightmares, their queen (Princess Nightingale/The Nightmare Queen), and Blade.
Chapter 1: Dabbling in Dark Magic
Everything you knew about Luna's imprisonment on the moon was a lie. Well, not everything. She did, after all, rebel against her sister, Princess I'm the perfect dictator, look-at-me-and-my-beautiful-sun. In case you didn't figure out who that is, it's Princess Celestia. They, meaning Celestia, didn't tell you everything about Luna's banishment to the moon. There was a third princess. This is her story. I'll just come out and say it, I am Princess Nightingale, Queen of Nightmares, and I am partially responsible for Luna's banishment. I suppose that's enough background for now, so I shall begin telling the story.
Once upon a time, there were three princesses. One was not a princess for a very long time. She was originally an ordinary pony with four friends. Before the mane six that everyone in Equestria came to love and adore, there were five friends that were never cruel to each other. Sure, they had their fights, but so do many friends. The five of them had the elements of harmony. One of the friends had to carry not one, but two elements of harmony. This would cause problems later. At some point, Discord attempted to take over Equestria. These friends were assigned by Celestia to stop Discord from taking over.
Discord was clever, but surely you knew that. He had been watching these ponies for weeks to discover what would tear them apart. He decided to lead them to the hedge maze. Does that sound familiar? Oh, how history does repeat itself. Anyway, while the friends were wandering the maze, Discord planted a seed of doubt in their heads.
Seeds of doubt, while sounding like nothing more than a figure of speech, are actually a spell, and a very dark one at that. What they do is just that, they make you doubt. You notice nothing at first, but you start doubting your relationships and, as a result, acting colder, and sometimes meaner, towards those you care about.
The aforementioned friends defeated Discord, but he never truly left their minds. They bore the emotional scars of what happened in the maze, and, little by little, started believing more in the seed of doubt's delusions rather than reality. They would fight more often than not, and were jealous of their friend, the new Princess Nightingale. She was the friend who bore the element of magic and the element of kindness. For her performance in magic, she was titled Princess of Equestria.
Because of her royal duties, she grew further from her friends. In a desperate attempt to save their friendship, she would organize gatherings for her and her friends. When that didn't work, she tried to use dark magic to make her friends feel happy. Her attempts proved to be only temporary. Eventually, she and her friends had an enormous fight and started tearing each other apart emotionally, and that was the end of their friendship. Princess Nightingale was devastated. The seed of doubt planted inside her began to turn her coat into a dark grey.
Princess Luna was concerned about Princess Nightingale. She noticed that Nightingale would barely speak and was quite literally turning darker shades of grey. Luna tried to comfort her, but in the process, was corrupted by Nightingale's delusions (and the fact that Nightingale had at some point planted a seed of doubt into her head). Eventually, they were plotting against The Empress of Light *cough* I mean, Princess Celestia.