Once upon a time, in the ancient and wonderful kingdom of Equestria, there was a beautiful castle, located on a place called the Everfree, surrounded by a lush forest. That forest was filled to the brim with playful and kind creatures. Even the most intimidating ones were kind and gentle. On the castle, lived the Rulers on Equestria, the Alicorn Sisters, Princess Celestia, the older one and represented by the Sun, and Princess Luna, the younger one and represented by the Moon, along with several servants. The people of Equestria admired and loved their princesses. However, the people tended to pay more attention and to adore the Solar Princess, as they called her, more than the Nocturnal one. At first, it wasn't such a big thing. But, with time, the younger princess began to feel depreciated. And, slowly, her heart became more cold, distant and selfish and she also tended to lose her temper easily. Eventualy, this reached the point that she refused to attend the few subjects that she still had. She didn't went to parties, barely left her room, didn't mingled with society, making her awkward when she sopke to others, and rejected any visitors. Especially if they were there to see her sister, Celestia. She didn't resent Celestia. No. She was her sister after all. But she had lost any trust and faith she had on their subjects.
On one snowy night, a ball was being held. And Celestia had managed to convince Luna to give their subjects another chance and attend the party. However, once in the party, none of the subjects talked to her or even aproached her. This, of course, was like a stab in the lunar princess heart. She got up and returned to her room. Once again casting herself in loneliness. Celestia saw this. And it broke her heart to see her beloved sister suffering so. But she decided to let her go, thinking that she needed some time to calm herself.
Once the ball was over, Luna came back down, altough she wasn't very keen on eaving her room. Thus, not being in a particularly good mood. Still, while she didn't mingled with their subjects, she still was gentle and, somewhat, friendly with the servants. Who had always been there for her and her sister. Suddenly, there was a knock on the door. Seeing as Celestia wasn't around and the servants were too busy cleaning the aftermaths of the party, she dicided to attend the door herself. Upon opening the door, she saw an old Unicorn woman. Her clothing barely covered her, she had many missing patches of her coat and several wrinkles around her muzzle. Luna also noticed that the elder was holding a rose on her left hand that, altough it was beautiful, didn't catch Luna's attention.
"Please", said the old woman with a shaky and low voice, "Would you accept this simple rose in echange for shelter against this cruel and bitter cold?"
Despite the condition of the poor mare, her humbleness and her pleading tone, Luna wasn't moved at all and looked at her with cold and indiferent eyes.
"A rose?", she said with a tone as hard as steel and with eyes as cold as ice. "What do I need a rose for? Leave this place and do not bother us again!"
Luna then slammed the door on the old unicorn's face. She was walking back to the stairs that lead to her room, when a bright flash of light appeared on the middle of the looby. Blinding all present. Luna then felt a strong force covering all fibers of her being. As did all on the castle.
"You have been decieved by your own cold heart.", spoke a beautiful but imponent voice. "A curse upon your house and all within it! Until you have found one to love you as you are, you shall remmain forever, as a beast!"
All servants, as well as princess Celestia, were transformed into diferent objects. All according to their offices. Celestia was transformed into a carriage. Luna tough, felt her everything change. Her midnight blue coat turned as black as a starless night, her eyes became teal colored with slithed pupils, not unlike a dragon's, her teeths became sharper and the incisive bacame full fledged fangs, her clothes morphed and became armor pieces that covered her head, hooves, wrists and chest and her cutie mark changed from a white crescent moon surrounded by a patch of blue to a light teal crescent moon surrounded by a patch of purple. Luna screamed all the time, as the transformation was painful. And the rose that Luna had refused, was placed by this strange force in a crystal dome upon a desk on Luna's room. So as to remaind her of what she had done, what she hadn't done and of her fate.
The castle itself turned from a warm and welcoming looking place into a somber, black castle, with terrifying Gargoyles on the roofs that looked as tough they would attack at any moment. The gates that were always open and free of any impediment closed and grew black iron and steel bars that ended in spikes. Discouraging anyone who may try to go inside.
The outsides of the castle didn't remained unchanged. The forest lost it's green luster that had remained despite the snow and became a dark and somber place. The trees lost their leaves and looked as tough one simple touch would break them. The lakes dryed and the animals became twisted. Even those who were pacific by nature, had tunrned into ferocious creatures that would attack anyone without a second thought.
"From this day onwards, until you find one to love this you, you shall be known, as Nightmare Moon."
And so, the people distanced from the castle, and soon enough, only the stories of the castle and their inhabitants remained. Many of those stories were forgoten throughout time. However, one, above many others, always remained. The story of the Cursed Lunar Mare. And that story, along with others, was pased from the elder ponies to the new generations. And so it was for a thousand years. Celestia, as well as her's and Luna's servants mantained their hope. They prayed for a miracle. For the curse to be broken. The Lunar Princess, on the other hand, had long lost hope of the curse being broken. For who, could ever love, a beast.