Author's note: Please note, this particular chapter was not in the original plan for this piece. It's effectively an optional or alternate ending, which got into my head after reading some of the comments on the piece. It's still perfectly valid, though it changes things around significantly.
So if you don't like it, pretend it never happened. :) /lack of conviction
Luna smiled, as the colourful trail of Celestia's tail blended into the window, leaving shadows in their wake.
So... that's what it feels like...
Her smile cracked within seconds of Celestia's departure. It was all she could do to keep her feathers from shaking. Though she still had far more experience than any mortal pony, she had nowhere near as much as her sister, and Luna was certain it was only the distraction of the impending meeting with Twilight that had kept Celestia from seeing right through her.
Well, if one were sufficiently paranoid, perhaps the magic of the hall might have helped.
Luna felt more than sufficiently paranoid right now.
"I... I guess I'm going to have to get used to that, then." She said to herself, trying to keep the tremor out of her voice.
"Get used to what, then?" She nearly leapt out of her skin as the voice came from right next to her ear. She turned frantically, spotting the flash of brown but it always spun with her, until finally she growled, conjured a mirror from the air, and glared at Discord - whose head was poking through her mane as if it truly were a portal into space.
But even with this gross violation of her space by one such as he, she couldn't keep up her annoyance, and just sighed. "Lying. Lying to people I care about, about the things that matter most to them, right to their face. Breaking promises and betraying the elements. Just like my sister. And directly to my sister."
"Aha, so you've decided to go along with the plan, then?" In the mirror, Discord slithered out of her mane and through the reflective frame, leaving them together in the room again as he leant on his front paws with a smug grin. "I knew you'd see things my way."
"Oh, as if you weren't listening to everything she just said." Luna snarled again, but quickly slumped, looking utterly defeated. "Seeing you laughing with her was one thing... but to imagine I might be conspiring with you against Celestia..."
"It's hardly against Celestia if it's being done for her own benefit." Discord said, with a grin wider than his own muzzle. "What she wants most out of this is a happy ending. A thousand years of giving everyone else those endings, you don't think it's fair she gets one as well?"
Luna glared, taking a threatening step towards the Draconequus. "And what do you get out of this? Why are you so eager to see her happy? I'm still far from trusting you."
"What do I get? I get to play with Equestria!" He spun around, taking in the hall around him in a grand gesture. "And I'll get to play with her as well, naturally. Don't you worry a hair on your little he-- well, a star on your little head anyway, little Luna. I've learnt my lesson. Friendship and magic and harmony, woo!" He waved an excessively tiny flag with the symbols of the elements emblazoned on it. "I don't have to be the villain to have fun here. And after all, I'll have you to keep me in line, won't I?"
Luna groaned at the thought, covering her eyes with her wings as images of the centuries to come impressed themselves on her mind. She uncovered them quickly, however, when the next words out of Discord's mouth didn't have his usual mocking levity, but sounded quite serious.
"But we can both agree that the best thing we can do - for your sister and Equestria and ourselves alike - is to go with our plan, isn't it?"
The look of distaste on Luna's face was exquisite, as she desperately sought any other option, any reason to not go along with what Discord suggested. She truly had wanted to stand behind her sister, when she believed she was doing this to save Twilight, and Equestria.
Discord's plan had been tempting from the beginning, of course - the trickster spirit always knew what buttons to press, though at first she had rejected him outright. And she had promised her sister she would stand by her decision, that she would do what Celestia wanted...
But in the end... this really was what Celestia wanted, wasn't it?
She sighed.
"Let's do it, then."
~-~-~-~
The windows, the hall, the stories were a powerful magic, and they protected themselves, as Celestia had mentioned. There was always a way out, right up to the last moment. Sometimes, the way out got unpleasant, as a way to discourage the guide from ever moving down that path again.
A millenia and change previous, Celestia had come close to breaking the stories to save Luna, but had backed out at the last minute, and the result had been a thousand years without her sister. Perhaps that could have been significantly lessened if she had stuck to her guns from the beginning. Luna didn't know what the way out that Celestia had seen for this moment was, but she feared it would be worse still.
But the hall offered another way out - just, not offered it to Celestia. Indeed, for Celestia, it was more of a way in.
Luna and Discord stood before the window Celestia had disappeared into, and focused their power. The magic they cast was a fiercely complicated yet instinctive work, fuelled by their own powers but guided and shaped by the hall itself.
Luna's strength lay in dreams, in metaphor and prophecy, connection and desire, and here in the hall that power was at its strongest. And Celestia had just stepped into the window to greet her student - physically walked into the representation of the stories. The metaphor there was a strong and obvious one, easy to utilise.
Discord's strength lay in change, taking the properties of one thing and giving it to another, or changing something small and letting the difference cascade out. Within the stories his power lay mainly with changes to the physical, stretching to the mental and emotional if desired; but from here he had a grip on destiny itself - and while he could fling it around without care for the details leaving devastation in its wake - as Ponyville had seen - he had more than enough ability to do so more completely as well.
And Celestia wanted nothing more than to leave the hall behind, and see her happy ending.
If she had not desired it - or indeed, if she had even been aware of the plan at all - it almost certainly would not have worked, for the powers of the hall would normally only work on those within the stories, not those who wielded the hall's power itself. But with all the three of them together, the same goal in mind even if their paths were different...
It was a little too convenient, Luna thought. It all came together so perfectly, the pieces falling into place. Perhaps we're not as far outside the stories as we thought.
Perhaps this is how the stories truly protect themselves.
She realised she never did find out what happened to Celestia's own mentor. Maybe this is how all of the hall's inhabitants ended their wardship of the windows.
For a second she was tempted to break off, to stop the change out of spite, but only a second. Her sister had waited a thousand years for her happy ending, and now she could have it, one way or another.
As the spell came together, she met Discord's eyes, nodded, and pushed the magic forth.
The window solidified. In Ponyville, miles away, two Alicorn princesses materialised back into the world proper, one having gained a great deal, the other...
She let out a breath, looking at the image before her of Twilight, wings spread wide. A destiny rewritten, with no trace on the window to suggest the other destiny that had been overwritten beside it.
"How will it work?" She breathed, looking at Discord. He shrugged, looking uncharacteristically sober.
"No idea. My guess is, she probably won't remember most of it. Just enough to justify her decisions throughout the centuries. Prophecies she came across to thwart, patterns she recognises from experience. The hall, the pressure, the worry... she won't remember them."
Luna looked up again at the window, sadly. "Thank you."
"I could have left hints and dribbles that would have driven her insane, of course, but it wouldn't have worked if we didn't have her best interests in mind." He grinned.
"Happy endings." Luna nodded.
"Besides, tormenting her will be so much more fun if she's at her best."
"Just... give her a little while, alright? Prank me if you have to, but leave her alone for a bit." Luna whispered. "Just... I want to see her happy for a while. Really, properly happy, not just putting on a smile, for once."
"Oh, you know she'll never really be happy unless she's got a problem to fix. And I'll be more than happy in turn to oblige!" Discord laughed. "But I suppose I can go on a short holiday. This place is going to be getting ridiculously regimented with the coronation coming up. Bo-ring! I assume you can hold the fort?"
Luna nodded vaguely, still staring up at the window. Discord rolled his eyes at the lack of response, sighed, and clicked his fingers, vanishing.
Luna just stood there, still staring up at the window, the magic of the hall still strong, but even now she could feel the difference without her sister being tied to it. She knew it was only the beginning of a long road she had stepped on - without Celestia's support outside the stories, and with her strength within them, the tales to come would likely be far more worrying yet - though her sacrifice had ironically left her in the perfect position to nullify the tale of crystal darkness with Twilight.
But right now, she stood alone in the hall, tears trailing down her muzzle, thinking only of how she had cut herself off from her sister. After everything that had happened, she was actually the strong one, now. She had what she had fought for, so long ago - perhaps not the respect and love, exactly, but the power and focus that Celestia had achieved before her, that had been central to her jealousy and downfall.
She'd taken the power of her sister, and now sat alone where her sister had undoubtably done so a thousand times before, knowing she couldn't share it again with her sister - not without ruining everything she'd just done. The symmetry was just another suggestion that even the hall's keepers were still enmeshed in the stories.
But similarly, Celestia had finally gotten what she had fought for, for so long. Only time would tell if she'd made the right choice in the long run, but for now at least...
"Happy endings, big sis."
Luna smiled.