I will not let you down...
It has been long since the Changelings have invaded Canterlot, and most of the damaged building had been successfully repaired. They had seemed to flee back into the desert country somewhere, disappearing suddenly. Nopony really knows where they went after all of this time, but both Luna and Celestia are glad that the kingdom is back to a peaceful state of being.
The day was starting to grow dark, the warm day being flooded by the cool night air. Celestia was retiring to her quarters for the night, hoping to be left alone for a peaceful rest through the night. She lay down in her bed, sliding the bedsheets and comforters over her body, resting her head on the CloudCover© pillow. As soon as she closed her eyes, a deep blue light started to seep through her eyelids.
She curiously opened her eyes to see where this ominous glow was originating from. Floating at the end of her bed, was a rolled up piece of parchment that had been partially charred at some places. She tilted her head slightly, grabbing the parchment in her magic and bringing it closer. Strangely enough, it was sealed with Luna's personal seal, something that she had not seen in a matter of ages. She tore the seal, revealing the words inscribed within.
Dearest Sister,
I am sorry to say, but I must be leaving you now. I have failed to protect those under my wing, and I have let many innocent ponies get hurt. You may never know of where I was during the invasion, and it is my wish that you never know such a fact. I had my business to take care of, and the whole invasion may have as well been my fault after all. I know that you had wished to have precedence over all democratic relations to other people, Celestia. I took matters into my own hooves with the Queen of the Changelings, and that is something that I truthfully regret doing.
She had said that she was here for peaceful relations, hoping to have new friends in a far land. What she did, however, was something that I did not expect to happen. I did not remember any relations with a group such as the 'Changelings', but then again I had been banished to the moon for some thousand years. What I did had the purest of intentions, I assure you, but I must leave this land. Be aware of the 'sister' that walks by your side, Tia, she is but an imposter.
When I had originally met with Chrysalis, she had introduced me to her daughter, a princess of their people. She had requested that we touch horns, as it was a 'custom', and I obliged in my ignorance. You had told me to always refuse such a request, but I thought this different than what you had once told me. I thought that they were a peaceful nation, but as soon as we crossed horns, I blacked out. I don't remember anything but a copy of my personal being looking over me, draining all the love and magic from my body. I couldn't do anything about it, and I'm sorry I have let you down after all you have done for me.
I am weak now sister, and all I ask of you is to rid of the imposter that lies in your company. She is but a deceitful liar, and nothing more. They wish of harm far greater than that of what had occurred at the wedding. They want more than just love to feed off of Tia. They are a parasite, and will embody anpony that become weak enough to overtake. You may not even know it, but there is a strong possibility that many of those you hold dear are but Changelings. Do not fret though sister, I am going to right what I have done wrong to you. Be aware, however, through the draining of my magic I have become a mere mortal pony once again, and to rid you of all changeling interference in your life. I wish you to live well and rule the kingdom as best you can, and rule it with the iron hoof you always have. I do not wish to disappoint you, and so the destruction of these parasites will cost me my life.
I will always be here, sister, as long as you believe that I am. My spirit will stand next to you, should you allow it. Even though we have been through our own set of obstacles, we were able to conquer them together for the most part. You may have banished me to the moon, but there was no shame in what you did. The mare you banished was Nightmare Moon, and that is something that everypony needs to understand. They need to understand that what you did was for the best of Equestria, and it was even the best for me.
Hesitation is something that you do not usually show, Tia, but I fear that among receiving this letter that you will be filled with a fear deep within your heart. There is nothing to fear. I will make this right, and the only action required by you is to act as you normally would. When it is time to rid of the plague from this land, you will know. I will be there, and you will feel me pass on. Be strong, this fear, this mess that I have contracted upon Equestria will be cleared and all will be restored to its proper order.
My absence during the initial invasion is highly regretted upon, I had been taken, and most of my love had been drained. The pony that impersonated me left to go there at a later time, leaving me in some sort of cage. I was able to break out of it, and I still roam through the hive. Now is my time, Tia, and now is the time to fix all that is wrong.
I love you.
Your faithful Sister,
Luna
The letter dropped to her bed, and with a light scuff it fell to the floor. Celestia sat in her bed, shocked at what she had just read. It had to be her sister, there was no there was no way for anypony else to know of her seal, she thought.
“I've been waiting a long, long time for this,” a scratchy voice called to her as it opened the door to her chambers. “You are all mine now, and your sister is gone. How joyous of an occasion when you can rid of a royal sister!”
The creature appeared to be Luna, but its appearance slowly started to change into a black form about the size of one of her guards. At that moment, two of Celestia's 'guards' walked into the room, slowly morphing into a black shape that blended with the dark night around them.
Celestia felt a slight pain in her horn, Luna's true voice in her mind. I will not let you down, sister.