Present Day
Celestia's horn flashed, and in that brief moment, two couches appeared.
"I've always liked the view from up here," she commented casually. No doubt an effort to break the tension. The castle's network of spires and archways laid out beneath them. She climbed onto the sofa. "Yes, its your turn to tell a story, otherwise I fear you'll only hear my side of it. As princess it is our duty to hear all sides of every story."
Luna approached her. A rare grim look on her features. "I will enlighten you, but before I tell you a single word of my side, I want you to hear something." Without further warning, she touched her horn to Celestia's. Celestia would suddenly feel an emptiness unlike anything she'd ever felt before. It was pure and overwhelming loneliness. A feeling as if one had grown up with no friends, no family, not a single pony worth caring for. Immense sadness, complete isolation, not a single spark of laughter. On top of that... she was completely deaf. Every sound, every rustle, everything was cut out. It lasted only a few minutes, but it was terrifying. The experience seemed to last for hours. When the feeling lifted and sound returned, Luna spoke quietly.
"Imagine feeling like that every moment of every day, for a thousand years, when not even your own voice can break the silence."
Celestia looked shaken, composing herself, gasping. Looking sad for her sibling. "It… it was like that for you, wasn't it? I'm sorry for you to have gone through something like that, Luna." Reaching over she nuzzled her sister. "But you forget that I sang to you when the moon was in its zenith. I knew my sister hadn't disappeared into Nightmare Moon. And you forget that you slept a lot there. Though... now that I've felt it, it must have been hard for you to know when you were asleep and when you were awake. Do you remember what I sang to you?" Celestia asked. Gently, tenderly, her voice broke into a heartbroken chorus.
"...May these thousand winters,
swiftly pass I pray..."
Luna harmonized, and her voice matched her siblings, the two sharing a rare moment of song.
...I love you, I miss you.
All these miles away..."
The two alicorns bowed their heads, noses touching, and there was complete silence between the two of them as each grieved in their own way for the actions of the past.
"Share my burden, Celestia, and allow me to enlighten you," Luna said quietly. Luna sat next to her sister, and touched horns again. It had the same effect as telepathy, only stronger. She was able to share the memories that Celestia had missed. In that flash of a moment, they were young again, and running away from an unknown evil.
The Dark Days
Celestia looked at Luna, fear in her eyes. "Run, Lulu! Teleport as fast as you can, and don't look back!" There was another flash forward, a surge of light, and Celestia only saw the flat surface of the moon, the inside of a particularly large crater. There was nothing, nothing but dust and rocks. A single dark blue hoof burst out from beneath the surface, and a battle ravaged Luna tore herself out from inside the crater. Her horn glowed, barely functioning, but just enough to keep her alive.
"Celestia?" She called, tears in her eyes. "Celestia!" She screamed, she screamed until her voice was gone. But in space, nobody can hear you. Nobody can help you. Luna's voice seemed to disappear the moment it left her lips. She cried until her tears were gone. But nopony came to help her. The memory broke away to present day, where Luna was crying again.
Present Day
"It was horrible, Celly. I never wanted to back there. How could I have possibly known what my jealousy would become?"
Celestia was dumbstruck. Her eyes blinked. Then she simply sighed.
"I'm sorry that this happened to you. I should never have asked you to jump as far ahead as you possible could. Of course you would have ended up on the moon, you're intimately linked to it. That place is your natural home." She never said anything about regretting sending her there the second time. But it was clear she had regrets, in her own way. She had done what was necessary, and it had its consequences both on the moon as well as in Canterlot. "Did you use the moon's power to get you home?"
"Yes." She said. "I was fortunate that there was an eclipse approximately two months after landing there. With the suns power weakened and the moons power increased, I was able to harness my horn to send myself as close to you as I possibly could. I think that's when you found me." She explained. "I never want to go back there, Celestia." She said again. "It's a horrible place."
Celestia considered something, mulling it over. "We no longer have the power of the Elements of Harmony on our side as we once did.That broke when I banished you. And was lost when we passed them on to a new generation." Her eyes flashed and a blue crystal was summoned. "I am, however, able to store my powers over time. This... should do for a very long teleport for the two of us, there and back." She stood up from the couch and approached her sister. "The first time you got there you ended up in the worst possible way and you were separated from me… the second time, you were stuck even deeper inside the rock. Do... you want to visit the moon with me, just the two of us? I have never been there before. And we'll make it a good visit this time."
Luna's answer was immediate. "No." She said. "I refuse go back there." But after a moment, although her eyes were still teary, she relented. "Maybe." She said. "But you must allow me to return home whenever I want, even if only after a few seconds. I have nightmares about the moon, Celestia. I'm scared. I'm scared of being alone."
"Little Moon Pie," Celestia said affectionately, as if her little sister was scared of the dark. "...you know I'd be right at your side all the time and as soon as you go home I'll follow you. No pony is left behind alone there anymore. This time we'll try not to end up inside a crater." She floated the crystal which positively vibrated with an inner energy between, putting her hoof on it. "You know the moon, you steer the teleport spell, land us on the surface, I know you can do it."
"I know the moon like the back of my hoof." She said. "The moon did not always have craters, you know. I had to find something to do over those thousand years. Imagine trying to dig to Saddle Arabia through the middle of the planet with a spoon. That's what it was like." She sighed. "I will go, but I want you with me, every second of every moment." She insisted. "There is not much to see, anyway. It's all the same."
"My morbid little sister, I'll be with you every single step, I'll even let my wing rest on your back just so that you can feel me." Celestia kept her hoof against the crystal. "You need to do this Moon Pie, the moon is… is like my sun is to me, and I can't go there." A genuine sense of longing in Celestia's voice. "Come, I want to see you there, and in a good way this time. It may be the only way to finally banish these demons of yours."
"We cannot stay enormously long, the moon will need raising here, and I cannot raise the moon FROM the moon. I've tried." She apologized. She closed her eyes, took a deep breath, and placed her hoof on the crystal. Teleporting to the moon was different than teleporting in general. They reached incomprehensible speeds, speeds that Rainbow Dash herself couldn't dream of. The ride took mere minutes, with the two landing inside a cave deep inside the moon, a cave once dug by Luna herself. There was only one way out, a single tunnel. It was completely dark, and turned into a scene from a horror film upon illumination. The name 'CELESTIA' was carved into the walls of the cavern, hundreds of times, over and over and over again. Words that were not meant to ever be seen.
Celestia's lips moved, but no sound escaped.
"..." then she realized her mistake and giggled silently. There was no sound, no feel of air, they could expand their lungs, but feel no air coming in from their nostrils. Any flap of a wing would be meet by no resistance. Her horn glowed and magic allowed a voice to be where none should be "I take it this was the tunnel Nightmare Moon carved?" She shone a light into the gnarly tunnel that went upwards in a steep spiral. Celestia took a bit more firm action and put her head against Luna's rear pushing her out of the cave and onto the moons surface. It was a tranquil basin. White powdery dust in curved hills, and rocky mountains. "..." Celestia mouthed again and turned on the horn
"Its beautiful, Luna."
Glancing back at the tunnel, Luna felt she owed her sister an explanation of her madness. "Do you know what the worst part was?" She asked. "The worst part of a thousand years of being by yourself?"
Celestia rolled her eyes. Then she heaved Luna upwards sending her in away in a parabolic arch, jumping up to follow her. "What, little sister? I'm not going to trivialize the torment you went through. I'll probably never understand how horrible it was. Luna, you're on the moon, with me, are you going to spend the rest of your time still reflecting on what Nightmare Moon felt here, or what you feel?" She landed on her back, wings sprawled, though the height of their jump had been large, the landing velocity was low. A quick burst of dust from their fall fell almost instantly in the void.
Luna had never spoken of what she admitted next. Never hinted, never admitted, never let Celestia know the depth or consequence of her isolation. When she finally spoke, her voice was full of shame. "Between the corruption of Nightmare Moon and the instability of centuries of madness, I... I forgot your voice. I forgot your name." She hung her head, tears falling from her eyes. "The sun always shines for you, Celestia. You always know of its warmth and count on its return. You... you are my sun, Celestia. And my sun went out."
Celestia reached over and nuzzled her sister. "Even if you had forgotten how to walk and talk, I'd have taught you it all again. Now I just want to see my little Moon Pie again, who was always badgering me about all the wonderful things about the moons she had learned. Who wanted to ask me what I thought the moon really looked like. Now that I think about it... you never asked those questions after you came back. Do you still love the moon?"
For the first time in a long time, Luna started laughing. Her laugh was musical, like wind chimes, and was a sound rarely heard. "Yes, of course. I always somewhat resented living in Equestria, because it only has one. Did you know some worlds have five or six moons?"
"So much responsibility, or do they move on their own there" Celestia smiled, happy to see her little sister laughing genuinely on her own. "Do you know what I sometimes hope?" She asked, with a little of the wonderment she had when she was young. "I hope that when Twilight and Cadance are ready, we can move on… not to the place where our little ponies go, but, you'll be this moon and I'll be the sun. Do you ever dream... of being the moon?" She sighed. "Or perhaps I've gone sentimental over the millenia."
Luna just gave a soft smile. "I want to be cremated." She admitted, again something that she had never said before. "I can think of no better fitting final pledge of loyalty to my sister than to go out in a solar flare." She shook her head. "But that time is not near to us, we have much to do before then."