Black

Luna walked to the balcony in her tower. She saw Celestia was setting the sun, the sky turning a soft red as the sun dissapeared behind the horizon. Luna then raised the moon, trying to ignore the jolts of pain that came as she lit her horn. The sky softly turned into a deep blue as the moon rised, until it was a deep purple. The stars slowly appeared as the moon was at the top of the sky.
Satisfied, Luna went back to her room, locking the balcony door.
After checking that every window and door was firmly locked, she threw herself on her bed. She took off her jewelry, putting it in her night table, pain surging through her horn as she did. The pain had become worse than yesterday. "This is no normal headache. What is going on with me?" Luna asked herself.
stop talking to yourself like you expect an answer, she thought.
She laid herself on the couch, taking out a bottle from her wine closet. It wasn't alchohol, like her sister had thought, but a potion she had bought on the black market. It seemed it was the only thing that numbed the pain. Pouring the yellow liquid down, she finally got a clear head. She started to consider the options.
This headache could be the sign of a disease, though if it was just me getting sick, painkillers should be working.
Besides, with a normal headache, the pain isn't at the back of the head, and it doesn't disturb magic like mine does.
No, this is clearly some sort of magic. Which means somepony did this to me! Who would dare such treason? Luna thought angrily.
Well, I am going to find out. In a few hours, my midnight court will open. Perhaps the culprit will be there.
Luna smiled. "Whoever you are, I will find you," she said.

***

Luna sat on her throne, ready to open the Midnight court. "I declare this Midnight court OPEN!" She used the royal Canterlot voice on the last word. It had been a while since she had used it. She didn't remember it to be so fun to use.
A noble came in. Luna recognized him instantly. Jet Set. "What is your business tonight, Jet Set?" She asked. He bowed before answering.
"It is important. Celestia wouldn't listen-"
"If my sister didn't see it as important, how do you expect me to be different?" She asked harshly.
"W-well, you see, me and my wife, Upper Crust haven't got our invitation to the Gala this year. Could you by any chance help with that?" He asked. "I'll look for any reason. Wait here," Luna said, leaving for the archive. She looked through the guest list to the Gala.
Neither Jet Set or Upper Crust were on the list. Then she looked on the list of banned guests. She found them instantly.
Jet Set: Reason for ban: tax evasion
Upper Crust: reason for ban: tax evasion
Luna instantly teleported back to the throne, trying her best to ignore the pain. The potion was wearing off.
"Jet Set, you and your wife haven't got your invitations because of tax evasions. EXPLAIN YOURSELF!" Luna shouted, outraged.
"Why should we pay tax like common ponies? We are too rich to bother with such things," he replied, trying to keep a steady voice. Luna facehoofed. "Well, if you are too important for taxes, then I guess you think you are above the law. Tax evasion is a serious crime, the sentence five years in jail, which is where you and your wife will spend the next five years. Take him away!" Luna ordered her guards.
"Wait, I can give you money, lots of money, just please, not jail!" The stallion pleaded.
"And that is five more years for trying to bribe a princess. NEXT!" Luna said, her guards dragging the struggling stallion out of the courtroom.

***

Luna spent her time in the Midnight court looking for the culprit who had caused her headache. No such luck. It also seemed like most ponies who came to court were trying to get away with crimes. The rest of them were pleading to get an invitation to the Gala.
Is this really what the Midnight court has been reduced to?

After the last pony left, she returned to her tower. The potion's effect had vanished near the beginning of the court, and she had snapped at several ponies. She lied on her couch, trying to track any kind of spell that could have caused her such pain. No such luck.
perhaps it is just a normal headache. No. No normal headache would cause pain in the horn. There must be magic involved.
Luna opened her wine closet to look for the potion. The bottle was half full.
After a quick sip of the yellow liquid, Luna teleported to the Canterlot archives. She started looking through books on dark magic, looking in the section on curses. There were several spells that gave the effect of headache or nausea, but none that hurt the horn. There were also speels that hurt the horn or prevented the use of magic, but Luna knew she was perfectly able to cast magic.
Frustrated, she went to the section of magical injuries. Looking through all the books, she found a book explaining magic headaches.

A unicorn can often feel a sharp pain in their head and horn if they have cast a very powerful spell above their limits(see page 601), or after having a trauma after having a dampener spell cast on them(see page 265). It can also occur if the unicorn in question has used dark magic(see page 500), suffered mind control (see page 278), been posessed (see page 304), or after experiencing a major change in magical energy. (see page 42)
Luna read this through a few times to see if she had any of the symptoms. It was true that having a shared dream with everypony in Ponyville had been quite taxing, but it clearly stated on page 601 that if that was the case, she would feel it as soon as she finished her spell. It said the same on page 500, 278, 303, and 42. Frustrated that she had come no closer to the truth, she teleported to her tower, and started yelling in her pillow. Not only because she was mad, but also because the effect on the potion had wore off, and teleporting had made her head explode in pain. When she finished screaming, she picked up the potion, with her hooves this time, and took a sip. It didn't work as effectively as it used to, so she drank more. It didn't help. She also saw that her coat had become a little darker.
"Probably just my imagination," she said
"Oh my, it is time to lower the moon," She stepped out, and looked towards Celestia's tower.
The sun princess was already in the process of raising the sun. Luna began casting her spell, trying her best to ignore the pain. But it only grew. But Luna didn't stop. She had to lower the moon. But she couldn't finish the spell because the pain became unbearable. She tried and tried, again and again, multiple times, until she fell unconcious, still trying to cast the spell.

White.
Luna woke up in a bright room on a bed. Still half asleep, she looked around. She realised she was in a hospital.
"Luna! Are you all right?" A familiar voice asked. The voice sounded distant, like the pony who spoke was far away. Still, Luna recognised the voiced.
"T-Tia..." She muttered.
"Oh, Luna. I noticed the moon wasn't going down, so I went to see you, and you were lying unconcious in your room. I got you here as soon as possible. You're going to be fine, Luna," Celestia said, although she mostly tried to convince herself.
"The moon.."
"I took care of it. But it is not important right now. What is important is that you will be fine," Celestia answered.
"Tia, h-help me.. It h-hurts," Luna managed to say.
"What hurts? Luna, tell me."
"Head.." Luna answered before everything faded once again.

***

Luna woke up to the night sky, the pain gone. She couldn't help but notice how perfect it was. The moon was at the top of the sky, every star perfectly placed, the colour of the sky dark but inviting, and she even thought she could see other celestial beings.
"Beautiful, isn't it?" Somepony said. Luna turned around and to her horror she saw "Nightmare." Luna was prepared to fight, standing in a fighting position. "Oh please, there is no need for that," Nightmare Moon said, walking closer. Luna noticed that Nightmare Moon didn't have her armor. "What do you want, Nightmare?" Luna asked.
"Isn't it obvious? You know how you have been in pain lately? That was me. You may have thought that I dissapeared when the elements struck me, but I have always been here in a corner of your mind. After the tantabus incidinent, your mind was weak enough for me to try to break free. But you have been resistant, my friend. It hasn't been until now I have been able to speak to you. Let us become one again," Nightmare moon requested.
"Never! I will never let you create eternal night. Why do you want that anyway? Everypony will die without the sun," Luna asked.
"I can easily recreate plants to get energy on moonlight instead. Come on now, Luna. It will be the only way they will respect you," Nightmare explained.
"Never!"
"Do you really want to continue this way? Never invited to the gala, feared by the nobles, ponies coming to your midnight court to try and get away with crimes or request what Celestia has denied?" Nightmare moon asked.
"The answer is NO!" Luna answered.
"Fine. If you won't do it willingly, I WILL HAVE TO USE FORCE!" Nightmare took hold of Luna, Darkness swirling around Luna's body, growing into a sphere, slowly consuming her. Luna screamed. "HELP! SISTER! ANYPONY!"

Help...
***
Celestia refused to leave Luna's side, even though the nurses pleaded her. "She needs to rest. Please understand, Princess," a nurse said.
"No! I will stand by her side until she gets better," Celestia replied.
Suddenly, Luna stood up. "Luna! You're awake. Are you feeling alright?" Celestia asked.
"Y-yeah. I feel alot better. I think I can leave now," Luna answered, stepping out of bed.
"Princess, are you sure that you are alright?" The nurse asked.
"Yeah. Never better," Nightmare Moon said.