A thousand years. The number rolled around in Luna's mind so many times it didn't even sound like ponish to her after a while. But the number was still there as plain as day, and there was no getting around it. She may have returned to her beloved homeland, and her sister would be by her side as she always was, but there was no ignoring the fact that this was no longer the world she once knew. Even this place, this Canterlot in which she now stood, was something new and grand that she'd have to get used to. But, let it never be said that Celestia didn't do all in her power to make her sister feel welcome in this bizarre place. The chambers that had been set aside for her after her return were, in many ways, an exact replica of her quarters back in their old castle in the Everfree, right down to the arrangement of the books on the shelf. Luna was grateful of course, but even with this comfort blanket of familiarity, she knew there was much work ahead for her. However, for now, she put such thoughts aside, as there was a sudden knocking on her door. "Yes?"
A couple of seconds passed before a voice could be heard on the other side. "Princess Luna? May I enter?"
It was a mare from the sound of things, and Luna considered the request. She still wasn't the most social of ponies, but she had just spent the last ten centuries by herself. And since she'd hardly spoken to anypony besides her sister since arriving in the capital she silently decided that it might be time to start making some connections. So, after giving a small nod that she only realised in hindsight that nopony would see, she answered. "Yes, please, come in."
She heard the creak of her door behind her, as well as the sound of hoofsteps, and after a few moments more, she took in a deep breath, ready to start her efforts to reintegrate with ponykind, turning to greet the new arrival. "So, how may I help...you?"
She had trailed off as she'd spoken those words, for what she now saw was probably the last thing she'd expected. Her visitor was, as she's predicted, a mare. But that was where the expectations ended. She was tall, with a largely pink colouring about her, and undoubtedly the most noticeable thing about her was that she bore not only wings, but a horn, just as Luna did. After a long and uncomfortable silence, this mystery mare finally spoke up again. "I'm sorry for disturbing your solitude, but...I feel like you and I needed to meet."
After staring at her for the longest time, Luna gave a solemn nod. "Yes...I believe we do."
The younger alicorn glanced around the room briefly, spotting a table and a couple of chairs by the window, which she now gestured to. "May I?" She saw Luna nod silently to her, accepting her request, and soon the two were walking towards them together. They got settled in, a feeling of surprise, confusion and just general awkwardness permeating the room by this point. After thinking hard on how best to start this meeting, the pink mare finally just blurted out the obvious. "So...I'm guessing you have questions?"
Luna did indeed have questions, and a plethora of them, all swirling around in her head. After a short while though she elected to ask the most basic of them. "Who are you?"
Her fellow alicorn gave a bow of her head. "My name is Mi'Amore Cadenza."
"Bit of a mouthful," Luna remarked, not really thinking before she said it.
Thankfully, this Mi'Amore wasn't concerned, giving a quick giggle. "Well, most of my friends call me Cadance."
With that ice-breaker out of the way, Luna, at last, got down to the meat of the conversation. "You...are an alicorn."
Cadance's earlier smile faded, replaced with a more serious look. "Yes, I am. I imagine that must be a surprising thing for you to see."
Luna looked away briefly. "It is. For many centuries prior to my imprisonment, my sister and I were the only such ponies in all the world. And when I returned, I expected that to still be the case." She looked back to her visitor. "For another of our ilk to be in the world...is an unexpected development."
Cadance chuckled drily. "Trust me, it was a shock for me too when it happened."
Luna tilted her head in a sign of curiosity. "Pray tell, exactly how did it happen? My sister and I accomplished our change through great magical feats, the raising of the sun and moon. What was it you did to change yourself, Mi'Amore?"
Cadance sighed. "Well, it certainly wasn't any plan of mine, if that's what you were wondering."
"I imagine it wasn't." Luna replied. "Even so..."
Cadance looked to one side, raising her hoof and tapping the side of her cheek. "It's...a long story."
"I've no doubt. But since I am in no need to be elsewhere, take whatever time you need," Luna said softly.
Cadance smiled, then looked deep in thought, recounting old memories that were no doubt among the most important events of her life. "Well, it all began in the village I had called home for so many years. Evil was taking hold there. A powerful dark magic that was stealing love from others. I went to face the one causing it, and after coming out as the victor...here I am."
Luna nodded slowly as she considered that. "An act of heroism against formidable dark powers. Yes, it makes sense that a transformation such as this might be the consequence."
Cadance shook her head. "I'll be honest, Princess Luna. I'd lived a fairly sheltered life until that point. I didn't even know alicorns existed until I became one. And when it happened, my whole life changed. I met Princess Celestia, took up residence here in Canterlot, it...it was all so overwhelming."
For the first time today, Luna smiled. "I can imagine. I still remember when Tia and I changed from what we were. Our appearance, our power, having it all be so different and strange was a terrifying experience." She looked straight at Cadance. "But when we realised what it meant and what we could do with that newfound power, we knew we had greater purpose. Greater responsibility."
Cadance too smiled, giving a nod of agreement. "Well, there's no doubt about that. Ponykind entered a time of peace it had never known once you two entered the picture."
Luna's smile faded. "For a time anyway. Until...me."
Cadance knew she'd struck a sore spot, and so her voice became appropriately apologetic. "I'm sorry, I didn't mean..."
But Luna shook her head. "No, it is no trouble. I erred, Princess. And my sister did her duty to protect Equestria from a tyrant with dark designs." She took a breath. "And for a thousand years, she led them well. Guiding them as a gentle mother over her children." Her smile returned as she thought on that. "I am truly proud of my sister for what she has done these past centuries. Giving Equestria such peace and prosperity."
Looking to her, Cadance too regained her smile. "You can help with that now too, you know."
Luna sighed. "Perhaps. Though I doubt other ponies would agree." She glanced down to her own hooves. "I appreciate them making a big show of welcoming me back to my old self back in Ponyville...but my actions cannot be forgotten. I was a monster who tried to usurp their beloved princess and bring eternal night. I nearly killed Twilight and her friends. Forgiveness will be a long time off. And wanting me to actually rule alongside Celestia? That will take even longer."
Cadance actually smirked here. "Maybe not as long as you think. Ponies are a forgiving bunch after all. As for your crimes of the past, most ponies nowadays see Nightmare Moon as a myth, a fable, something to tell stories about to colts and fillies to get them to behave. There is even a whole day to commemorate it."
That caught Luna's interest immediately. "A day of Celebration? For my dark self?"
A nod from her fellow princess. "And one of the most popular occasions of the year. So believe me when I say that, if you were to approach anypony nowadays and ask them how they feel about Nightmare Moon, hatred is the last thing you'll get as a response."
Luna stared out of the nearby window for a long time, mulling over those words. "I wish I could believe that."
Cadance reached out, placing her hoof upon Luna's. "Thing's are difficult right now, I won't deny that. Both you and your sister have been through a lot of dark times together. And I'm not going to lie to you and say that your return is going to be without bumps in the road. But I promise you, whatever fears you have, there will come a day when you can step outside those doors and walk among other ponies as though you never left."
Luna finally looked back to her, offering the smallest of smiles. "Equestria is fortunate indeed to have one such as you as one of their princesses." She arched an eyebrow. "Tell me, what domain do you reign over?" She placed her hoof upon her own chest. "I rule the night. Celestia rules the day. What of you?"
Cadance's smile widened. "Love. It was that which I fought to defend before my ascension, and it is that which I now stand as the guardian of."
Luna took a moment to let that sink in, then gave a nod of acknowledgement. "Then I would say love has itself a fine guardian indeed."
Cadance smiled in appreciation. "Thank you."
As before, Luna looked out of the window. "I have a long road, it is true. Perhaps a day will come when I can step out and walk among my beloved ponies once more, as I did in ancient times. Perhaps there will also be a day when my dark actions of the past are but a memory for me, instead of the stain that they are today. But those times are still ahead, and for now I feel I need to contend myself with simply adjusting to how much this nation of mine has changed in the centuries of my absence."
The Princess of Love chortled after hearing that. "Then I'd say you've got your work cut out for you. From what I've heard, the Equestria of your day was quite a different animal to what you'll find today."
Luna looked back to her. "Even my brief stint in Ponyville proved that much to me. A village that did not even exist when I last experienced freedom." She considered her own words for a time, then smiled. "A village that will no doubt have more than its fair share of thrills in its future, now that my sister's apprentice resides there."
Fondness came to Cadance's expression after hearing that. "Twilight? Oh yes, that young mare has quite an exciting future ahead of her, you mark my words."
Luna looked to her with curiosity. "You know her?"
A proud nod from the other princess. "I should say so. I was her foalsitter back when the two of us were younger. And her brother serves as my sworn shield."
Luna tapped her chin. "Such a young age...yet already closely associated with two alicorn princesses." A knowing smirk came to her. "I see my sister's penchant for long-term chess games has not dwindled during my leave of absence. I know a plan of hers when I hear one."
Cadance giggled. "You're not wrong. You should hear Auntie Tia talk about her whenever she receives one of her assignments."
Luna raised an eyebrow. "Auntie Tia?"
Cadance blushed with some embarrassment. "Oh, yes, it's...um...just what I came to call Celestia after a while."
A moment of pause between the two alicorns, broken when Luna again smiled to her, reaching out and patting her on the hoof. "In that case, welcome to the family, Mi'Amore." After seeing the other mare return the smile, Luna continued. "You said your friends call you Cadance? May...may I also do so?"
To that, Cadance beamed, as if waiting to hear that all day. "Princess Luna, I'd be delighted."
Author's Note
I had always wondered how Luna might have felt to discover that being an alicorn was no longer something only she and her sister could claim to have known. Feels like it would have made for an interesting interaction in the show 
