"Luna," Princess Celestia said as she knocked on her sister's bedroom door. "Are you in there? I have to talk to you."
"Come in, Celestia," Luna answered. The princess slowly opened the door. Luna was sitting at her desk reading a book.
"The Crystal Empire," Celestia began grimly. "Has returned."
"W- What?" Luna asked as she closed her book.
"The Empire is back. I sent the guards to send Cadance and Shining Armor, and I just sent a letter to Twilight."
"Do you want me to-"
"No. Stay here for now. I don't want anything to happen." Celestia cut off her sister's sentence. "I will be in the main hall. I do not want you leaving Canterlot until this gets resolved." She opened her mouth as if she wanted to add something, but instead left the room and slammed the door behind her.
Luna got up from her desk and walked over to her bed. She pulled a key out from in between the two mattresses and used it to unlock a drawer on her nightstand. A tear rolled down her cheek as she pulled a book from it. She opened the book to the first page, and was greeted by a heart shaped piece of paper that was delicately taped there. Scrawled in black ink on the heart were four words. Four words that had started it all. Four words that kept Luna alive on the moon. Four words that made her hope, and four words that made her cry.
I love you, Luna.
Luna remembered the day King Sombra had given her that paper heart.
...
She was at a fair. She was riding rides, and playing games, and eating cotton candy. Then, she saw him. A dark blue unicorn stallion. Luna stared at him for a moment. He was talking with a tan pegasus and eating a pretzel. Then, he looked at Luna. Her heart leaped inside her chest and she slowly, inadvertently began walking towards him. The pegasus punched him on the shoulder with a smug look on his face and trotted off.
"H- Hello," Luna stuttered. "I'm Pr- Pri-"
"Princess Luna?" he asked. Luna nodded. "I didn't think I'd meet a Princess. Especially not you, your highness," the stallion said as he bowed his head. "I'm Sombra,"
"Pleasure to meet you," Luna said as she regained her courage. This is the one, Luna thought. This is the stallion I'm going to marry someday.
...
Luna flipped through the book as she recalled the glorious memories of her very special somepony. Then, about half way through the book, the memories stopped. The rest of the pages were blank. Luna knew what happened, why the memories stopped. Celestia had discovered her sister's love for Sombra, but she had seen darkness in his heart. She forbade Luna from ever seeing him again.
...
"No! You can't do this!" Luna screamed at her sister.
"Luna, I told you. There is darkness inside of him. He will turn on you," Celestia said angrily.
"I don't care! I love him! You can't make me stop seeing him!"
"This is the last time I'm going to tell you this, Luna. You need to listen to me! I forbid you to-"
"No! I won't listen! I love him! I- And I hate you!" Luna screeched at her sister. Celestia was obviously hurt, but she tried to hide it.
"Fine! I can live with that! But I swear, if I ever hear of you being with Sombra again, I will banish him!" she yelled as a tear streaked down her face and landed on her throne.
"You can't do that!"
"Yes I can. I am in control of the land."
"Then- Then I'm talking control! I will not lower the moon in the morning! Night will be eternal and I will have total control over Equestria at last!" A cloud of black smoke surrounded Luna, and when it cleared, a wicked creature of the night remained.
"L- Luna?" Celestia asked.
"Don't call me Luna. I am Nightmare Moon!"
...
Princess Luna cringed at the memory as her own harsh words echoed through her brain. The terrible memory continued through her thoughts as she was reminded of her thousand years on the moon, and of how she had many times slipped through the black fingers of depression on the hope that someday she would get to see her beloved Sombra once again.
Then, the princess's eyes filled with tears as she remembered when she came home.
...
"Where is Sombra?" Luna asked her sister about a week after she returned from the moon. Princess Celestia cringed before answering.
"He... he was angry. He nearly killed all the ponies in..." Celestia trailed off as she started to cry. "I'm sorry, sister! I knew you would be so sad, but I had to! I just- I- he-"
"Celestia!" Luna cut her sister off from her ramblings and put a comforting hoof on her shoulder. "Take a deep breath, and tell me what happened." Celestia took a few deep breaths before she told her sister the entire story.
"He came to Canterlot looking for you. I told him what had happened, and he got angry. In his rage, he overtook the Crystal Empire and declared himself king. And... he... changed." Celestia levitated a book from a secret shelf and flipped to a page near the back. The page showed a drawing of a black unicorn with purple smoke coming from his eyes and a red curved horn. He was wearing armor and a red royal cape.
"Sombra," Luna whispered.
"He nearly killed all the ponies there. I did what had to be done. I transformed him to shadow and banished him to the arctic north. But he cursed the Crystal Empire to vanish for one thousand years."
"When?" Luna whispered.
"What was that, sister?" Celestia asked.
"When did this happen?" Luna yelled.
"Shortly after you-" Celestia was cut off by a loud sob from her sister as Luna grabbed the book and ran to her bedroom.
Luna did not come out from her room for a long time. She cried most of the time. She was terribly depressed. She didn't know how she would live without Sombra. She never even got to say goodbye. The last words she had ever said to him were "I'll see you tomorrow," but that was a thousand years ago.
"I miss you, Sombra," Luna whispered as a tear fell onto the picture of the dark unicorn king. "I love you."
...
Princess Luna put the book back into the drawer on her nightstand and locked it. She didn't care what Celestia said. She was going to the crystal empire to see the one and only pony she ever loved. She began walking to her bedroom door when she remembered what Celestia had said.
"As if I'm going to listen to you, Celestia. May I remind you that it's your fault he took over the empire anyway. You kept us from each other, but no longer! I will see him, no matter what!" she mumbled heroically to herself. Slowly she bent down and looked through her keyhole.
Celestia, of course, had stationed two guards on either side of the door frame.
"You think you can keep me here that easily, Celestia? You are mistaken!"
Luna pushed open her shutters and stuck her head out of the window into the warm, afternoon breeze. She climbed up onto the thick windowsill. Soon, she was going to see the stallion who she had loved so dearly all these years. She couldn't wait to escape from her sister's iron grasp and see her forbidden special somepony.
Slowly, the princess of the night leaned forward and jumped off the windowsill, spreading her wings and smiling.