Once upon a time...
There was a little shepherds boy, who had the reputation of being incredibly smart... and folks said that he had an answer for just about everything, his fame spread far and wide for the wise answers he gave...
Luna stared out the window at the Nightmare, her gaze lingering. She rushed about, trying to figure things out, before she realized it was following her. Rushing into a hallway, she ran to the door at the end finding it locked. “Locked!” Luna growled, “And a complex-spell inhibitor… Thou art smarter than I gave thee credit for,” she muttered. With one quick wave of her horn, she used her magic to push the right pins and unlocked the door, only to find a stone wall.
One day, the Emperor heard about this phenomenon and called for the boy. He wanted to find out for himself because he could scarcely believe that a mere shepherd's boy could be as smart as all that was said. Telling him that if he could answer three questions… he would be treated as the Emperor’s own son, and be able to live in his palace...
Luna crashed into the water, skittering from the force before plummeting deep within. Passing out, she drifted slowly… until the lack of air jolted her senses, and she saw the multitude of skulls…
Wanting to test the waters, the Emperor looked the boy in the eye. “First, how many drops of water are in the ocean?”
Luna shook herself off to the best of her ability as she clambered out of the water, her body heavily waterlogged. A sharp lance cut through her head as she remembered something, but all she could really focus on was the thought of all those skulls…
The shepherd's boy thought it over, and the Emperor looked on in interest…
Luna grabbed the spade in the courtyard and began to dig. “Of course you would do that, Celestia,” she muttered, and she slowly began to whittle away at the earth…
“My lord, have every river in the world dammed so that no drop can run off into the ocean before I have counted it. Then, I will tell you how many drops of water there are in the ocean.”
“No, that can’t be…” Luna murmured, gazing at the stars. Her next shovel hit gold and she brushed away the dirt. “We are the Nightmare… we are in the Moon?”
Amazed, the Emperor smiled and went on to the second question…
“Hello?!” Luna cried, having found the room with the Moon, “Is anypony there!? Can anypony hear me?”
The Emperor asked the shepherd’s boy “The next question is: how many stars are there in the sky?”
“If I didn’t know better, I’d say I’d travelled a few hundred years into the future,” Luna whispered to herself, “Who’s been playing with my stars? Who’s been playing with my sky?
“Who has defiled my moon?”
The boy asked for a great sheet of white paper, with which he began to dot it. Almost one on top of the other…
“You know? I hated her! I hated Celestia! Everypony loved her, and they all shunned me! Me!” Luna shouted at the Nightmare.
When the paper had been filled with dots, he took it back to the Emperor, and said “There are as many stars in the sky as there are points on this paper. You just have to take the simple task of counting them.”
“No… I can’t be evil enough to have the elements of harmony used against me!” Luna begged, watching events re-unfold before her eyes…
And yet, no one could do it. Just one glance at the paper could take away their sight. Impressed without a doubt, the Emperor had one last question for the boy…
“Come to play, have you?” Luna remarked, facing the wall with resignation. The Nightmare remained behind her, shambling it’s way to it’s target. “Want me to tell you a story?”
The Emperor, with his last question in mind, asked the shepherd’s boy…
“...how many seconds… in eternity?” Luna said, and the Nightmare enveloped her. Her body wracked in pain, her mind shattering from the action…
And the shepherd’s boy replied…
Luna shakily pointed her horn at the crystal, and she released her remaining magic. Crying out in pain, the mana flowed into the rune, and Luna died… as a new one was born. Her life in the castle flew by, each confession, each admittance, each memory… burning it’s way across her mind… all the way to the wall...
“...there’s a mountain of pure diamond…” Luna breathed, waiting a moment before punching the wall again. it takes an hour to climb it… and an hour….” The wall had been degraded already a few centimetres, but not noticeably enough.
“An hour… to go around it!”
“...and every hundred years, a bird comes to sharpen it’s beak…”
“AGGHHHHHHH!” Luna shrieked, the magic being torn away from her.
She hacked and coughed as she appeared anew in the room…
She cried as a memory resurfaced, as old wounds were opened again…
Scoured the castle for everything it had to hide…
Always returning to the wall…
“...and every hundred years, a bird would come to sharpen it’s beak on the diamond mountain!” Luna shouted, a small tunnel now created in the wall. She leaned against the side as she exerted her own earth pony strength to help.
“And when mountain has been worn away by the actions of this bird…”
Luna found herself crying in the same places…
Digging the same holes…
Exploring the same castle…
Dying and living and dying and living…
All so she could finally punish herself for the damage she had inflicted to herself, to her subjects… and her sister…
Always leading to the wall…
Always wishing for the best…
And never remembering any of it for her next iteration…
“When this mountain has been chiselled away,” Luna cried, tears running down her face. “...The first second of eternity will have passed!”
“...the first second of eternity shall have passed…”
“Why is it always me?” Luna whispered… “Why must I be the one to be tortured, humiliated, ignored, and unloved?”
“Why can’t it ever be anyone else?”
The Emperor watched the shepherd’s boy with benevolence, seeing the wisdom this boy truly had to be remarkable…
“I’d say I’ve travelled a thousand years forward in time…” Luna murmured, “But who’s played with my stars? Who’s put a pony on the moon?”
And the Emperor regarded the shepherd’s boy with newfound respect…
“...the first second of eternity will have passed,” Luna shouted, punching the wall with all of her might.
And he now saw him as an equal…
“You might think that’s a really long time,” Luna shouted, crying in every sense as she broke the wall with every single hit of the hoof.
From that day forward, the shepherd’s boy lived in the palace, becoming a trusted friend and advisor to the Emperor…
“But I think that’s one bird I’d like to know!” Luna cried, leaning into each hit.
And they all lived happily ever after…
“Celestia, forgive me,” Luna whispered as she closed her eyes, ready to accept her fate... to be a failure... to be the little sister that needed guidance... and the sorrowful martyr. She was content with how things were. With all that she had done, she just wished she had been kinder and more forgiving, like the sister she might never see again...
And just then, bright rainbow light burst through the remaining wall in front of her, nearly blinding her. As she turned to look away, she watched in astonishment as the Nightmare shrieked, it's form getting torn apart by the blinding light. The wall around her began to spiderweb, and everything began to crack.
Luna could feel the power the light contained, and she embraced it.
"I break free!"
Her whole world shattered around her, and she found herself lying on cold hard ground. She struggled to get to her hooves, her legs sluggish and her movements slow... as if she had not used them in a millennia.
Am I dead again? Luna wondered, before she saw who was around her and what was happening.
Six ponies she did not know wore necklaces with what she recognized were the Elements of Harmony, and she couldn't comprehend what was going on. What was happening?
The only thing that mattered to Luna just then, however, was her sister.
"Princess Luna," Celestia called, and Luna looked up from where she was recovering, the ghost of the Nightmare's presence and her mind-shattering memories still burning through her. She looked up fearfully with shame and regret at Celestia, and even more longing than she had ever felt before. "It has been... a thousand years since I have seen you like this. Time to put our differences behind us. We were meant to rule together, little sister. Will you accept my friendship?"
Luna's heart broke at the sound of her sister's voice, a thousand years of regret evident as well... and Luna couldn't stand it. She couldn't hold back her tears, glad to finally see her sister... glad to finally be with her...
With a face full of surprise and incredible sorrow, Luna leaped for Celestia's embrace. "I'm s-so sorry!" Luna sobbed, feeling the warmth of her sister's fur, "I m-missed you s-so much, big sister!"
Celestia couldn't keep back her own tears of joy and happiness as she hugged Luna back. "I should be the one saying sorry... I should be the one to blame..."
"Nonsense," Luna sniffled, content with her position hugging Celestia, "It was I who was at fault... and I alone am to blame."
"Luna," Celestia began, but Luna just hugged tighter, never wanting to let go.
"No, Tia. I am sorry."