"You seem exhausted, my sister," observed Celestia. She sat on her throne in the castle of the two sisters when Luna entered the room.
Luna looked sick, her mane entangled and sweaty, as she approached Celestia with her head close to the ground, "I'm still having bad dreams. I don't understand why, I tried everything and they keep coming. It's been like this for weeks." She shivered at what all this could mean.
"I believe you still don't want to talk about it," sighed Celestia, "You need to share your feelings with somepony, it is not good for you to dam them up."
The younger alicorn looked briefly up. Celestia had a short feeling there was disbelief and hate in Luna's eyes, but she shrugged it off. Her sister sat down on the other throne and gathered her will to look more lively.
Celestia looked worried, "Should I take over your shift again?"
Luna nodded slightly before she answered, "I don't know what to do. I should be awake in the night, guarding my little ponies dreams. I just don't know what is happening to me."
The older princess looked sympathetic at her sister before she proceeded. "Let the day court begin," spoke Celestia with the Royal Canterlot Voice.
It was past midnight. A ray of moonlight entered Luna's room through the open window. Luna laid exhausted on her bed.
She was used to be awake during the night. To be asleep or tired during her night was something unheard of for the public. Still the princess' thoughts drifted towards the imaginary. Her breathing slowed when her eyes closed. She was asleep.
A few minutes later something crawled through the window. It looked like a racoon or a black cat as it crawled to Luna. The creature sat on Luna's bed, hesitating shortly before climbing on the princess. It laid on her chest, it's head extending to Luna's ears.
The creature began whispering, "Luna. She will betray you. She hates you. Celestia doesn't deserve her throne." - A wicked smile appeared on it's face as it syphoned the dreams that Luna's mind now produced, feeding itself on the negative feelings - "You deserve more. The ponies don't like you. They hate the night. You have to force it on them. They have to feel your anger. Kill Celestia. She is nothing compared to you."
It chuckled silently. The princess' dreams entered the realm of unpleasantness. She dreamed how Celestia left her alone. How ponies insulted her for her night. Not appreciating the beauty.
It began whispering again, "I will help you. Make it eternal night, let my family feed on your little ponies. Let the hate consume you. You are the moon and I am your nightmare. I will grant you great powers. Don't trust Celestia."
Luna's breathing became strained from the weight of the nightmare and she began to sweat. She dreamt of how powerful she might be, if she took Celestia's life. How everyone would have to love her night. Forever.
The mare was satisfied. It climbed of the princess' body before she would wake up. A few minutes it was busy knotting her mane and tail. Then it left the sick princess alone in her room. The thoughts have to sink in.
He would come in the morning. This was the last night. His plan would be proved successful, his family could feed on ponies dreams. No more hunger. The princess had interfered with them often enough, preventing them from being fully satisfied.
It would still be over an hour until Luna would wake up. The nightmare climbed in her room again. He looked pleased.
The body of the princess had transformed. Her fur was now deep black instead of blue. The wings had changed, now looking more like bat wings with feathers than the elegant wings of a pegasus. Fangs were showing in the corners of her muzzle. The horn had grown, doubling it's size in the progress.
The nightmare climbed on the transformed princess. He sat on her chest again, her breathing becoming strained again. The pony princess looked uncomfortable. It strechted it's head to her ears again, "You are ready. Kill Celestia. I will help you. Let the darkness consume the day. You are the nightmare's moon. I will ride on your back. Give you help and guidance. Kill Celestia. They do not deserve to live in the daylight. Destroy their carefully constructed lie; that they have control over their lifes, control over their dreams. Destroy their free will. Feed us."
It pushed her dreams into waking up. She needed to prepare for the battle.
Luna looked in the mirror. She had created some armor for herself. Her delusional sister would be defeated. She would rule. Her snake-eyes gazed over her beauty. She was perfect.
The princess caught something in the corner of her eyes. The impression that something sat on her back, "riding" her. She turned her head to her back, ready to destroy any creature that tried to degrade her. There was nothing. Only the feeling something laughed at her stupid moves. She scanned the room, expecting to light anything on fire just with the sharpness of her eyes. Empty. Nopony there.
In her fury she fired concentrated magic to a vase near the bed. It shattered from the impact.
Nightmare Moon left her premises, ready to confront her sister and let eternal night rule. She laughed.
The nightmare hid in the shadows. He didn't expect Celestia being able to defeat her sister. The darkness in her had been strong, but apparently not strong enough to defeat the elements of harmony.
It still smiled. The mare now knew the secret weapon of the sisters. It could wait for thousands of years if necessary. The guardian of the dreams was now banished to the moon. Its family could now feed freely. It smiled.
"They don't want you. They hate you. They are not your friends. Help me and I will make them your true friends. Help me feed my family. Celestia is in your way. Kill her. Kill Celestia, Twilight. You can be my twilight and I will be your nightmare. You can be Nightmare's Twilight."