If there was one regret that Princess Luna had now, it was that she never told Celestia how sorry, how grateful, and how adoring she was of her beloved sister. Now, she would never have the chance. It likely did not matter to Celestia, but Luna never physically told her that she forgave her sister for what she had done. She had long since forgiven Luna, and even apologized for her actions. But not once was Luna able to come forward and tell the truth; Luna was sorry. The guilt had been eating her alive for two whole days, ever since her sister was reincarnated into a new form. The princess was now nothing more than a sphere of light, a womb for Celestia's new body. All her accomplishments, her relationships, her memories, everything that was the Princess of the Sun was now lost, forced to be reborn with her.
Luna could only ever bear to leave her chambers when the need arose, such as to perform her new royal duties, or to raise and lower the Sun and Moon. It was there, in her chambers, at mid-day, that she felt a filthy, wicked presence enter the room. The presence of a being not bound by the laws of nature in neither form nor existence. She knew this feeling, but she had not felt it in over a thousand years. Luna rose from from her bed, exiting the room and traveling to the Royal Gardens, where she hoped if thing went awry, there would be witnesses. "Oh come now my faithful General, are you so naive as to think simply walking outside would save you from me finding you?" Discord revealed himself nonchalantly, slithering through the air and around the Princess' neck. "Begone from be foul abomination!" Luna's eyes glowed white as she sent a shock wave rippling in all directions, only to be blasted by her own magical energies as a see-through force field rebounded her attack back at her.
"Release me, wretched mistake!" Discord chuckled, barely restraining himself from bursting into laughter. "Come now Princess, you need me, just as much as I need you.. I can give you everything you want, everything you need.. A kingdom, far more fast than Equestria alone.. I can take the burden of the throne, you would rule all, but I would take care of all the boring things you loathe in your new position.. What do you say Princess? All you need to do.." He pressed a talon against her head. "Is remember what Celestia did to you.." Luna, enraged at his ignorance, blasted him against the field of magical energy with a single, super-charged bolt. "You know nothing of what she did to me. She welcomed me back into society, she took me back at her side! And when she was faced with death," Her Canterlot Royal Voice threatened to shatter the field, "SHE PLACED ME ON THE THRONE!" Discord danced down to the ground, brushing dust off of his form casually. "Come now Princess, you're stuck in here with me. And whether you like it or not, the 'other you' is coming back, I assure you." Chains of fire and molten magma burst from the ground, chaining the Princess and restraining her permanently. "When I first was released from my prison, I had become a tad... insane. Cotton candy clouds raining chocolate, among other childish things, were the most I could do with my abilities. But my sanity has returned.. As as my power." He approached Luna, who watched in terror as her coat was singed by the flames. "Now, I know for a fact that you're going to be difficult. But we'll see who prevails."
Luna was suddenly observing herself in the past, still bound by the chains. Hooks clawed at her eyelids to force them open, painfully forcing her to watch as her younger self was speaking to Celestia. "Sister, we had thought and hoped thou wouldst allow a further hour of night. Why must thou lie? Why must thou promise, only to break thy word?" Celestia lowered her head. "Luna, we hath spoken with our advisors. Even a slight variation in our Sun's time in the sky wouldst harm all of Equestria. We must not allow harm to our kingdom." Luna frowned. "Thou speakest "our" but sound of thy. Doest thy kingdom belong to the two of us, or to thee alone?" Luna remembered this moment well. She could distinctly remember Celestia's face, it was of anger, of prejudice. "Return to thy chambers. We shan't speak of this again." Celestia flinched as she saw Luna stomp a hoof, before exiting the throne room.
Luna was forced to watch the several days that lead up to the events of her corruption into Nightmare Moon. All of these events had been manipulated from afar by Discord, to create his perfect puppet, his perfect General. As the days passed in front of Luna's eyes, and as time went on, the chains would tighten. Then, when she had thought it would never end, she saw her most painful memory play out before her, as she slowly shifted in front of her own sister. "We tire of these games, sister. We hath watched as thou hoard the sunlight and banish my night. We hath watched as thy loyal subjects shun my night, and praise thy sun. Where art our praise? Our love?" Her mane became a darker color, as it shot outward over her mane and coat. "We believe that if the night were to last forever, thine subjects would hath no choice but praise us!" The physical transformation was one tenth of the way finished, when Luna found herself at the first vision yet again. "What trickery is this?!" Discord laughed maniacally. "All apart of breaking you." And so, Luna once again watched the long, drawn-out transformation over several weeks as her younger self became Nightmare Moon.
It felt like centuries since Luna had been trapped in this perpetual nightmare. Occasionally, she would shriek Discord's name, to no reply. Each time she watched the final, physical transformation, she would come closer to seeing the end-result, before being torn and thrown to the start of her downfall. She could eventually barely remember if she was coming closer to the end of the physical transformation or not. And all the while, her chains grew tighter, searing through flesh, grinding bone. Luna could barely scream any further, her throat was dry and sore from what was likely weeks of screaming. Only the occasional whimper escaped her throat, and though the effort was always being made, it was impossible at this point.
It was inevitable. She would be broken, and she knew it.
Finally, at long last, she witnessed her past self become Nightmare Moon fully, in mind, spirit, and body. The chains released her bloody, seared flesh, her form collapsing to the ground, limp and barely concious. "Now, I've been very generous. Not only did I once, long ago, give you the power to overthrow your 'beloved' sister, but even though you failed, I gave you a second chance. And you threw it away. But, guess what? Go ahead, gue- Oh, heavens me.. I forgot you're so happy that you're just speechless!" He slithered through the air, drawing closer, a talon brushing her coat, now filthy of sweat and exhaustion. "I put forth the effort to break you, to bring you back to the winning team. I could've simply torn the remaining fragments of Nightmare Moon from your soul and made her pure. But no, I truly do like you, Luna."
He whispered in her ear. "Those chains were once a sort of.. Back-up plan. They belonged to Celestia. And they could only be used in a special emergency. Now, what could she possibly have wanted those chains to-.. Oh.. Oh dear.. Luna, Luna my dear, do you think it's possible that she had them in case you ever got out of hand?" He pulled away, patting her on her stomach like a dog laying on its spine. "You see, Luna.. She trusted you enough to give you her entire kingdom while she was 'in dispose'. But she NEVER could've trusted you enough not to have something ready. Just in-case. But I'm sure that, just like you say, she had forgiven you, right?"
Nothing could've held her back now.
The transformation shattered the barrier, which during her time inside, had become a crime-scene. It was the last place Luna had been seen since her disappearance nearly a month ago. A gardener had seen the Princess flying from her chambers, and landing in the Royal Gardens, where a strange force-field had grown around her.
And exactly one month later, as the barrier shattered in the middle of the day, ponies all around Canterlot shouted in confusion and terror as the Sun lowered itself without warning, without rhyme or reason, and a dark, sinister-looking moon rose.