"Princess Luna?" Whispered a confused Doctor as the alicorns passed him, one looking quite bruised. Midnight shot him a look that warned him to be quiet.
"It's alright Luna, let's get you to the palace," her wing wrapped around the princess as they made their way across the sleeping town, trailed by two unsure Equestrians. They reached the palace and were greeted by Illuma, looking startled at the princesses battered appearance. She led them up several flights of stairs the room that was prepared for when Luna visited.
"Can you guys wait outside? I need to talk with her alone," they nodded and the alicorns disappeared inside. Everypony waited patiently, except the doctor, for Midnight to emerge. When she did it was with a look of great sorrow.
"What's wrong?" Ditzy cried out before anypony else could speak.
"I need to explain, but please, let's do it in private," IIluma called a guard to watch over Luna as she rested then they proceeded to the throne room.
"I never knew how upset she was on this day,"
"What do you mean?"
"Ditzy, Doctor, you remember the day you first met?"
"Of course, it was the Summer Sun Celebration and Nightmare moon... oh,"
"Today is exactly a thousand years before that day. Luna has just betrayed her sister, and her entire kingdom, in her rage and she is devastated," there was something strange in the alicorns voice.
"But you don't think it was her fault," the Doctor spoke up
"No, I don't. I have wondered for a long time how such a kind, loving pony like Luna could manage enough anger to transform like she did. But everything she described to me, her feelings, her transformation, her powers..." she sighed " When I was a filly, before I met you two, I had a storybook. It told of many creatures who had been discovered or rumored to exist. There was one with only a single, unproved account. It was called a Vexion. It took ahold of a pony, feeding off of their negative emotions and amplifying them. I think this thing is the reason behind Lunas actions," her audience of three nodded.
"But why is she no longer Nightmare moon?" Illuma spoke at last.
"Well, I have two theories for that. Either the Vexion was ripped out of her by the elements of harmony, or simply that Luna has no negative feelings towards the Lunar ponies. It has no more food so it left her,"
They went to find Luna on her balcony. Midnight separated from the group to speak with her again before returning to Equestria. She explained her theory about the Vexion.
"Yes, that makes sense. Oh, I wish I could go back, apologize, even if it wasn't entirely my fault, but a powerful spell enforces my banishment and even if it didn't, I fear that thing would take control of me once again,"
"Luna, I can lift the spell holding you, but not now. In a thousand years time I can, and you must return to Equestria. The Vexion will return, you must know that, but know also that it will not be for long," it was all that was left to say.
"Midnight, are you coming?" she hesitated.
"Wait for me in Ponyville,"
"But how will you get back?"
"Trust me, I will," the two nodded and left. "Goodbye sister, I will visit you,"
"Celestia," the alicorn who had been weeping on her bed was startled. She was clearly weak from the battle and the stress of her lost sister. The cloaked figure hiding in the shadows frightened her in her emotionally weak state.
"How did you get in here? I-I can call my guards!"
"It does not matter. I have a message for you. There are already stories emerging in the country. Rumors of a sister who went bad and was banished to the moon, add this to these tales and take comfort in it for I know your sadness. The banishment will not last forever, your sister will return in a thousand years, her escape aided by the stars themselves. Know also that when this time comes you may not face her. Even now you can sense your bond with the elements has broken, others must be found to wield them,"
I made up the Vexion, it is a combination of 'vex' and'stallion' (because this is MLP), I have not read the commicsso they will have no affect on my story.