It was a late night in Ponyville, and reasonably warm for late autumn/early winter. Apart from this things seemed normal enough. Most lights were out, the few ponies awake in the small town indoors. One of the few lights still burning was in the library, as Luna had known would be the case.
Except... if she looked in a different way, to close her eyes and see with a second sight, there were many lights. Stars and suns glowing bright, lying in bed. She could see them for hundreds of miles. From the massive glows of Manehattan, to the smaller constellation of Canterlot, to isolated lights dotted around farms and country homesteads arcing all the way across Equestria...
To one light dimming nearby.
Luna had done this for centuries before her confinement to the Moon. One of her three duties as a Princess of Equestria. Few knew that one of the things Nightmare Moon had sought to do was end this duty. Few would understand how necessary it was. It would be hard to the average pony to understand. Luna understood it. But having only just resumed the duty in the past year it had not become easier.
If anything, it had become harder. To see lights dim all across Equestria with her second sight. So many each hour. More lights blinked into existence but she did not guide them. Her sister was their guide, but even a thousand years before the numbers of ponies had made it so that even within the power of their second duty they could not be there even in spirit for everypony.
It was sad to say it so bluntly, but only for special ponies did the sisters perform the duty in 'person'. When noble or important ponies became subject to one of the duties. Though for Celestia it was technically her first duty, she regarded it as secondary to raising the sun.
Sometimes Luna felt that her first and second duties were the same. Stars had to go out as well sometimes. Light had to dim in the skies too.
Luna looked for a last moment at the dimming light nearby, then looked at the world in normal eyes. The only lights now were her sky and the light of the library.
It was never any easier. But it was necessary.
Tonight she'd perform her duty for a special pony, and she would have to let other special ponies know why it had to be done.
Twilight Sparkle almost didn't hear the hoof knocking on the door with her ears, but there was a strangeness to it. A gentle knock, as quiet as Fluttershy, yet impossible to not hear. As if the knocking was not on the door but in her head.
"Who could that be?" She asked herself quietly.
Spike was asleep, and Owlowiscious was not very good at greeting visitors. Even if all he would do is ask 'who' as well. She had to get the door herself. She had not expected to see what she would see. At the door stood a tall mare, wearing a black cloak covering a greyish-purple body and blue mane.
"Princess Luna?" She squeaked.
"Hello, Twilight Sparkle. I'm sorry to say that I'm here on business, even this late..."
"W-Well, of course. Your duty is the night so it would make sense you're working right now... um...Please come in. I was kind of working too you see..."
The princess entered the library, horn glowing as she pulled the hood of her cloak down.
"I know. I apologise for the interruption of your work but... I'm sorry. I'm so sorry."
Twilight wondered for a moment why she had said sorry three times.
"...The other apology isn't for interrupting, is it?"
Luna closed her eyes for a second. Twilight did not know she was looking at a dimming light.
"No. It is for something that will be a great hardship to come, for you and for your friends. I have to explain to you what it will be and what it means, but before I begin I want to make it clear that what I discuss doesn't involve you right now. Not for quite some time... But you will be needed in the aftermath. You and your friends must be there for another."
"I'm sure we can handle it, Princess Luna!" Twilight said, with an honest smile.
"I hope you can."
Twilight felt a little nervous, but spoke anyway.
"S-So... What is it you want to discuss?"
"Twilight Sparkle, you are my sister's greatest student in generations. But do you know the duties of my sister and myself?"
"Of course. Celestia raises the sun, and you raise the moon and stars. Um, I don't mean to sound forward but you made it a nice night tonight..."
"Thank you. But those are only one of our duties each. Can you tell me what the three things that are guaranteed in life are?"
"Birth, Death, and Taxes." Twilight said right away.
"Correct. In fact Taxes are a third duty myself and my sister share."
Twilight paled visibly even below her fur.
"You... You mean... But I..."
"Twilight, I told you, this doesn't involve you. It involves somepony else."
"T-Then somepony I know is..."
"I'm sorry." Luna repeated.
"...How much do they owe?"
Luna blinked. During it she saw the dimming light once more.
"You misunderstand. Taxes are our third duty as rulers of the nation, but we allow subordinates to handle it. We aren't omniscient, certainly not when it comes to a bank balance. No, it is my second duty I am sorry to say I'm performing."
Twilight frowned.
"You raise the moon... Taxes are the third... So what's second?"
"What is my sister's first duty?" Luna asked.
"To raise the sun." Came the reply.
Luna shook her head.
"Her duties are at the dawn, that is correct. She guides the sun to set. In a way, she also does this with ponies. She guides them from their dawn to their sunset. Her first duty in essence is Life."
Twilight digested this, and her eyes widened.
"But then... Your second duty would be..."
"Correct."
"But it doesn't involve me... Then who? And why are you here to tell me?"
Luna closed her eyes once more. The light had grown dimmer.
"When my sister or I perform this duty, only some ponies can see us. Those who will see us when we perform the duty later in life, or some who can see other things. Most unicorns can at least hear us, but, well..."
WHEN THEY HEAR MY VOICE THEY HEAR IT LIKE THIS
Twilight shuddered.
"That was kind of creepy."
Luna continued.
"As part of the duty, I sometimes have to split myself. Technically speaking I'm not really here but at the same time I am, and in essence I will be. It's something of a paradox and is technically time travel but five future versions of me travelled back in time a small distance, so that they could explain to the other Elements of Harmony exactly what would happen. Technically you're the first though, as I've yet to travel back to let the others know from my perspective. I have travelled back after the event from Canterlot to be here, so that is why technically I'm not here. I'm also in six other places, but the correct me on the timeline is currently in the castle."
Twilight nodded.
"I understand. But you might have trouble getting some of my friends to... understand."
"In more ways than one. I sense that even though you know this has to happen you don't know why."
"No. I don't."
"It is like the sunrise and moonrise. A cycle that cannot be interrupted. To do so causes problems for everypony. Disturbing the balance. We cannot see the future far, nor do we have the means to determine fates precisely. I only learned this had happened a few moments before I travelled back..."
She closed her eyes for a second.
The light dimmed.
"...I learned now, in fact."
The light faded away.
"Nothing could have been done. To provide a warning of how it was going to happen would have changed things that were forseen already. I would have learned of something that would therefore not happen, and it would have distorted reality itself until eventually the anomaly would be corrected in other ways."
Twilight's eyes began to fill with tears.
"B-but... who is... where..."
"Twilight, I need you to be strong, along with your friends. I need you to go and support a friend who is going to need you. You will need your friends as much as they will need you this night and for many nights yet to come. I said I hoped you could handle it, because in all honesty, as I said, I cannot foresee if you truly can."
The purple unicorn blinked, but her eyes were still watery.
"I... I think I know what you mean. What do I do now?"
Luna went to the door.
"Go to Sweet Apple Acres. Applejack knows you and her other friends will be coming."
Twilight's shoulders drooped.
"Oh. She'll be heartbroken. I know how much she loved Granny Smith..."
Luna sighed sadly.
"It's not Granny Smith."
Author's Note:
A story inspired by some images of Princess Luna that popped up on Equestria Daily, combined with an over-active imagination perhaps. Perhaps some of you will know this but I dislike or shy away from permanent changes in stories sometimes. Personally, its because in most instances the change just doesn't work, but there is one change that I feel is a constant, that if you do it right, works very well. Life, death, taxes, the three constants of human existence. Everyone encounters them.
Boys, girls, men, women, the old, the young. Life always ends for us in the real world. Of course in fiction there are exceptions. Obviously, immortality or resurrection are two good examples. My personal take though is this change should, unless previously indicated by the in story universe that there is a possible exception, always stick.
In this case there is no indication of an exception, apart from that of Luna and Celestia. Storywise this doesn't connect to anything else and so has no exception.
Diligent reading hopefully isn't needed to realised that whoever the change affects is not answered in the story. Everyone should note that it is kind of open as to who Luna was here for, but in a way this story is a sort of spiritual successor to, or evolution of a prior story called DABDA in theme and a few other things. A hint or two lie here and there, but as for who it was? Don't look too hard for an answer in the story, you'll be better off looking here.