In her private bedroom; Luna started to don her black armor and boots before heading to her throne room. Accompanying her was her assistant; a lavender unicorn with dark purple hair. Their walk took them down a hallway, jokingly nicknamed the ‘Dark Passage'. Throughout the castle; the carpets were midnight blue to match their ruler; although that hallway had the addition of black marble lining the walls, along with stained glass murals of Luna in various poses.
“Come along Twilight,” Luna directed “No time to look around.”
“Yes; your majesty,” Twilight answered
As the two reached their destination; Luna headed towards the throne as Twilight moved to a position on the side. The throne consisted of a wooden frame with armrests carved to resemble pegasi wings held up by unicorn horns and topped off with a skull. The design would strike fear into all that entered the room; hoping not to suffer the wrath of Nightmare Moon.
“Send them in,” Luna stated, to a guard near the door
Twilight watched as a noble entered the room hesitantly before bowing deep enough that his muzzle was touching the floor.
“Guh-Guh-Greetings, y-your muh-muh-majesty,” he stuttered
“Get on with it.”
“I come on behalf of the citizens of Ponyville. They ask for a little more time to gather enough for your tribute. Since last week; they have said it would take at least another week to fully collect everything,” the noble explained
“Very well. Let no one deny that I am a generous ruler,” Luna mused “From now on; the tribute will be collected once a lunar-cycle for the same amount. They have until then to gather the money and food I asked for.”
“Thank you for your kindness,” he said quickly backing out of the room in a kneeling position
“Your Majesty; a band of rebels was recently caught,” a guard stated
“Kill them,” Luna ordered, dismissively waving a hoof
“No,” Twilight squeaked before Luna glared at her
“Did I hear that correctly,” Luna demanded
“I mean,” Twilight started, trying to find her voice “Perhaps wait to kill them until after they give up any others.”
“You have a point,” Luna replied “Tell Iron Will to have fun.”
“Right away Princess,” the guard answered
“I have a special favor for you,” Luna started once the room was empty “I’d like you to take a tray of food down to a particular cell.”
“If I may be so bold to ask; who is there?”
“I’m not at liberty to answer. If you’ll follow me; I’ll guide you partially there. I want to see some other,” Luna continued her adding with a chuckle “Guests.”
“May I talk to the prisoner?”
“I leave that to your judgment.”
“Maybe I’ll bring them something to read,” Twilight mused
“Shall we take our leave,” Luna asked
“Lead on; your majesty.”
Once out of the throne room; Twilight was trying to remember each hallway they passed to get to the kitchen. A portrait of Luna moving the moon in one hall; followed by another of her standing over a shadowy figure in the next. Standing at the end of the hallway in front of a door was a tall stallion in a chef’s outfit.
“Prisoner,” a chef demanded
“Yes,” Twilight answered hesitantly
“Take it,” he stated, shoving a box towards her
A small levitation spell was cast before Luna motioned towards another part of the castle. Nothing but solid black walls lined the area; making it entirely dark as Luna had her horn lit as a guide. At the back of the room was a staircase that spiraled downwards as Luna stopped at the first landing.
“Continue all the way down,” Luna directed “You can’t miss the room. Come up when you’re ready.”
Without a guide or source of light from any source; Twilight shifted towards the wall and kept her left front hoof on the side to serve as a partial guide. The lower she descended; nearly six levels down, judging by the number of landing she crossed; the colder the air around her got as she resisted her body’s urge to shiver. Not feeling the wall curve any more; she surmised this was finally the bottom.
“State your reason,” a voice called
“Food delivery,” Twilight answered simply “Also really cold.”
“Can’t help with that,” a second voice offered “Ready to go in?”
“I-I guess.”
“Yell and knock three times to signal your release,” the second instructed
“PRISONER; TO THE BACK WALL,” the first voice ordered
Twilight found herself being escorted through the door into a small space with a second door in front of them as the first was slammed shut behind them.
“SECOND DOOR OPEN.”
At this order; the door was only partially opened to just barely allow entry into the cell beyond. Just as she entered; her way out was effectively locked behind her; trapping her in with the prisoner.
“Food,” a new voice asked in a quiet tone
“Yes. I was told to bring this to you.”
“What are you?”
“What do you mean ‘What are you’? I am me.”
“Species. What manner of creature are you?”
“I’m a unicorn.”
“Would you mind using a light spell?”
“Can’t you cast one?”
“If only it were that easy. Instead of telling you; it’ll be better for you to see for yourself.”
Twilight concentrated for a moment as her horn ignited with the requested spell. Standing in front of her was a pony whose color was almost impossible to currently determined due to the amount of dirt and filth covering the body. Besides the fur; the biggest thing that drew her attention was a metal mask that covered the entire of the face; leaving the eyes and mouth exposed. As she stepped to side to continue the inspection; the masked mare lowered her head to show a discolored spot on her forehead were a horn would be have been; along with the fact that no mane or tail was present.
“My goodness,” Twilight said in disbelief “What happened to you?”
“Since you’re here; would you like to hear a story?”
“Of course I would. I’ve always been fascinated by books and tales.”
“Once upon a time there were two sisters that ruled the land. In their adolescence; it was decided they would split control of the sky with the eldest moving the sun and younger in charge of the moon,” the pony started
“Things started out alright with the two in harmony. In secret; the citizens decided to start to rally around the idea of only one ruler in charge of both the sun and moon and cults started to form dedicated to this.”
“The eldest failed to truly connect to her citizens to understand what was going on; just as her sister started becoming more distant. One night; while the eldest slept; a band of cultists broke into their castle with the youngest at the head.”
“Upon the capture of the eldest sister; the youngest immediately assumed control and had any that dared align with the eldest executed for treason. Wanting further humiliation; the eldest had her mane and tail burned off with a spell designed to prevent regrowth, her horn was hacked off with a cursed knife at the base to prevent magic use and the feathers on both wings plucked one by one to maximum pain before they were ultimately sliced off with the same knife. The flesh around the cutie marks was burned beyond recognition before a red hot iron mask was riveted into the skull to add to the torture and prevent any attempt at removal. She was then led down to this cell where they’ve been for at least a thousand years,” she finished
“What did they call you then,” Twilight asked, her voice barely above a whisper
“I don’t remember. It was stripped from me at that time and destroyed from all historical texts. No one has ever been permitted to speak of it since,” came the grim reply
“I-I,” Twilight started “I have to go.”
Trying not to be rude; she quickly slid over towards the door and pounded the required three times to attract the guard’s attention. Just as the door was cracked open; she ran through and pounded on the outer one.
“Just a minute,” the guard answered
Once out of the cell; Twilight began hyperventilating and needed to sit for a moment in an attempt to calm down. Not bothering to light her horn; she just took off up the stairs and quickly headed back to her room.
“I can’t believe anyone could do that. She can’t…” she whispered to herself, before shaking her head “No. Nopony can know. I’ll have to use a dream prevention spell.”
The next morning; Twilight was hoping to forget the horrors of what she was told.
“Princess; would you mind if I headed to the library?”
“Anything in particular you care to learn,” Luna questioned
“History. I’ve always wanted to learn more about our past.”
“There are a few books already in the library,” Luna argued “What more could you want to know?”
“Is it possible there are some occurrences that were omitted from the records?”
“She told you about something you want to see whether or not is the truth,” Luna accused “Be honest with me now.”
“Yes, princess,” Twilight answered with a sigh “She did tell me something and I was curious. Her claims felt unbelievable without some kind of evidence or explanation.”
“If that’s the case; then I give you permission to access the underground archives,” Luna decided “You are not to speak to anypony about whatever you may or may not come across.”

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I really like this story idea? Can I use it?

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Feel free. I would like credit in regards to the original idea if you don't mind.

https://www.furaffinity.net/view/20398506/

This was the inspiration I had for the mask

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Yeah, I’ll give you credit. I’ll try to get to this soon, and see if I can get something to work.