Chapter one: Came back.
I tried to lay on the floor to sleep, so this last day before my return could go faster. Memories of Luna and me, being happy, when we defeated Discord and King Sombra... A sweet dream, but really distant away as my beloved sister fell to her creation, wich used her hate and envy to make her stronger.
I open my eyes as I get up, my eyes looked at the Earth, as I trough about my creation: did my sister destroy it? I must hope that it didn't happened, because The Light is the only thing I can use to became more powerful and bring my sister back.
But when the words "Eternal night" echoed in my head, I began to think. How everypony survived all this time without the light of the sun?
"It's easy, Celestia" a cold voice said, appearing beside me my little but wicked sister, "Since the moon couldn't give, by itself, the necessary light for my subjects to grow food and other things, I used your creation to make my celestial object much brighter and warmer, so it replaced the sun" she looked at me, "Are you sure that you don't want them to suffer from using you?"
"I am, sister. And, believe my word, I will do everything to bring you back... Luna" my voice cracked as her furious eyes concentrated in my sad, pink ones.
"How many times I must remember you this, Celestia?" she asked, irritated, "Since that day, I am Nightmare Moon"
She didn't know how her words hurt me, but before I could say or do a single thing, a bright light engulfed me... Wait, this magic is so pure, and I know I saw this before... I am a fool! The Light, finally, was bringing me home!
"And don't you worry about it, Celestia" Nightmare Moon said, "You could still use your creation... if you reach it..."
"Is good to see you alive, Creator"
I open my eyes as I get up, this place was only whiteness everywhere... Slowly, a earth pony walked to me, he had white coat, grayish mane and green eyes, bearing the Cutie Mark of a grey void.
"Oh, The Light! Is so good to see you!" I trotted towards him as he embraced me, my tears slowly drying on his coat.
"I say the same, Creator" the pony smiled, but his expression became serious, "But I'm here not to talk to you, but more to advise you. Nightmare Moon is really powerful, and The Darkness is something you must fear"
"Why? It didn't seemed so strong..." I asked, more that just confused.
"Nightmare Moon trained us, and now it and I are the only things that can kill immortal alicorns"
I look at the floor and then him, my eyes letting the last tears fall.
"Do you know what Luna did to the Elements?" I asked, slowly cutting with my horn the bandages of my wings.
"Well, I don't know much... But I believe she has them in a secret cellar in her new castle" he looked at my pink eyes, "But first, you must find me. Without me, you could not purify the Elements of Harmony, or as Nightmare Moon calls them, the Elements of Nocturnality: Darkness, Nightmares, Dreams, Void, Moon and Stars"
I smiled, so do him. He began to vanish as he reached for my hoof, looking into my eyes. I kissed his forehead, knowing that it calms his mind and body. Everything became blurry again, as I disappeared, but I could hear one more thing from The Light...
"Is so good to know that you came back."
Well that explains how the world would survive Eternal Night. Though I wonder how a creature of Darkness like the Nightmare can corrupt or even use Light?
3691302 I trough I didn't explain what, exactly, The Darkness and The Light are... Well, Luna and Celestia created two things to maintain the balance not only of their nation, but of all the world: The Darkness (Luna) and The Light (Celestia). The Darkness corrupted Luna, so she became Nightmare Moon, and NM corrupted the Elements, to have them under her control. Maybe she used The Light, but being a corrupted alicorn and not it's creator, she couldn't control it: in other words, she can use it, yes, but for little things, because she will loose control of it.
I don't know if I explained my point very well; all I can say is that I hope I did...
3691360
It does. The main thing in my eyes is the explanation on how NMM kept the world from freezing and the crops from dying. Not many ever delve into that, Past Sins being on the most prominent that does.