When one thought of Princess Celestia, they tended to imagine her surrounded by opulent luxuries. Golden thrones, exquisite tapestries, and accompanied at all times by the highly-trained and disciplined members of the Canterlot Guard. What they didn’t usually expect was for her to be sitting quietly in a quaint little library in the middle of an out-of-the-way town on the edge of Equestria. But, that was exactly where she was, sitting comfortably in the main chamber of the Golden Oak Library of Ponyville, a contented smile upon her face. Carefully, she took a sip of the tea that had been prepared for her, waiting for her host to return. And sure enough, Twilight was soon seen walking down the stairs, smiling warmly to her teacher as she trotted over to join her. Levitating a small tea-cup of her own, the young unicorn promptly took a sip of it herself, before letting out a long sigh.

"I'll admit...I've missed this."

Celestia's smile widened.

"And I've missed you, Twilight. Since you moved here to Ponyville we've hardly ever seen each other."

A brief look of guilt came to Twilight at that.

"Yes...and I'm sorry about that. It's just..."

But Celestia raised a hoof.

"You needn't apologise. I realise how busy things have been for you. In truth, I think we were both lucky to even get this time to see one another."

Another sigh escaped Twilight, and she nodded in agreement, taking another calming sip of her tea. In the silence that followed, Celestia glanced to the side, to the staircase her student had just walked down.

"So...I take it Spike managed to get off to sleep fairly quickly?"

A smile returned to Twilight's lips.

"Yeah. But I'm not surprised. After spending all play playing with Sweetie, he gets pretty pooped."

Celestia nodded.

"I'm glad to hear he's making friends. And I'm happier still he's managed to find one among the Lunar people."

Twilight giggled to that.

"You should hear Cadence talking about it. The way she tells it, it's like Spike has done a better job forging a bridge between our peoples than she has."

The two shared a laugh over that, which ended when Celestia shook her head.

"She does herself too little credit. I think she's done wonderfully in her position."

Slowly, she gave Twilight her full attention.

"...And so have you."

Twilight blushed slightly.

"Oh...I just did what I could."

Celestia shook her head.

"Nonsense. You've done everything in your power to make sure things between our nation and theirs has remained stable. There have been...issues...but through them all...I think your efforts, more than anypony else, has kept things from getting as bad as they could have been."

Twilight thought on that, watching as Celestia continued.

"I mean when I say...that I'm proud of what you've done here, my student."

The young unicorn, naturally, was a little taken aback by such words, but even so, she knew to give an appreciative smile when people gave her a compliment. Again, a silence passed between them, and as Celestia occupied herself with her tea, Twilight, by contrast, began to look thoughtful. Something began to take centre-stage in her mind, and at first, she tried not to focus on it too much. But, Celestia was here, right in front of her. There was nopony else to overhear them or get involved. It was just a private get-together between teacher and student. Because of this, and after more than a little hesitation on her part, Twilight began to speak up.

"Um...Princess?"

Celestia looked up from her tea.

"Yes, Twilight?"

Twilight took a deep breath, then continued.

"You've...um...you've had other students, right? Before you started teaching me?"

And with that, Celestia's smile began to fade. The moment she'd heard her student's words, her expression of comfort was instead replaced by one of, of all things, resignation. It was as if that very question was something she'd been expecting, and one she was not looking forward to. This sentiment was further reflected in the long sigh she let loose, which was one that Twilight couldn't help but feel nervous about. But, she stayed quiet as her monarch finally responded.

"So...you've finally asked."

Twilight titled her head in confusion.

"I'm...I'm sorry?"

Celestia looked to her, appearing and sounding more serious than before.

"You want to know...about Sunset."

In that instant, it was as if all the colour was drained from Twilight's face, and she was once more a little filly, terrified about being found out about doing something wrong. It was a look Celestia had not seen on her in many years, but soon, her own face softened, and she spoke in a somewhat calmer manner afterwards.

"You needn't look so worried, Twilight. I knew about your going to see her...and I knew you'd one day ask me about this."

Twilight, understandably, didn't know how to respond to that. Shock and confusion and fear all wrestled for prominence in her mind, but, at the end of it all, she composed herself. Or, at least, she composed herself as best she could under the circumstances, before finally speaking up yet again.

"So...she was your student?"

Celestia nodded.

"Yes...she was."

Discomfort tainted her voice, but Twilight kept her mouth shut for the time being, giving her teacher all the space she needed right now. Celestia, for her part, gathered her thoughts together, trying to figure out how best to explain all of this. But, in the end, she began to tell her story all the same.

"It was many years ago...and rumours reached my ears of a young filly in one of the Canterlot orphanages. Talented and brilliant in magic, far more so than somepony her age should be."

Nostalgia came to her.

"I visited the place myself, and that was where I met her. Sunset Shimmer. The rumours had not been exaggerating. She was indeed gifted in magic in a way I had never before seen in one so young."

She took another sip of her tea.

"I decided to take her with me, back to the palace, and I endeavoured to train her as my apprentice."

Slowly, a smile came to her lips, and she looked up to Twilight.

"In the early days...she was like you. Always excited and eager to learn. She stuck close to me every single day, always listening with wide eyes whenever I had some new spell or lesson to teach her."

Twilight too smiled, remembering those days of her own education well. But, that smile, and that of her teacher, began to fade once the latter began to speak again.

"But then...as she got older...I began to see...other sides to her. I saw hints of pride...aggression...self-confidence that bordered on the arrogant."

She shut her eyes tightly.

"I saw...I saw my sister in her."

Twilight seemed surprised by this, but again, she said nothing, instead watching as Celestia carried on with her story.

"Luna too had been like that in her youth, in the days before our split from one another. And when I saw Sunset start to take those first few steps down that path...fear took hold of me."

Shame came to her.

"I feared what might happen if I continued to teach her...to help her power grow."

Twilight looked to her, unsure of how to take that.

"You...you were afraid of her?"

Celestia hung her head.

"I did...and that fear led to...one of my greatest mistakes."

Gradually, the mare of the sun turned, looking out to the window, seeing that bright orb setting beyond the horizon.

"One day, during one of our lessons together...I chose...to teach her...less."

A sigh.

"I would begin to hold back...teaching her smaller or weaker versions of spells I knew she was capable of learning. I...I held her back...out of concern of what she might become if I didn’t."

Her face twisted into a grimace.

"But...I underestimated her thirst to learn...her desire for knowledge of magic. One night, when everypony else was asleep...she ventured into the library, to the section restricted to all but the most capable of mages."

Her eyes narrowed.

"And there...she discovered the tomes I had been using...and found that there had been more."

Twilight's mouth hung open, for she knew where this was going.

"And...what happened?"

Celestia looked back to her.

"The next day...she confronted me. She asked me, flat-out...whether I had lied to her. Whether I had deliberately undermined her training."

Slowly, her eyes closed.

"...And I did not shy away from that. I confessed everything. I told her...all that she already suspected."

Shame returned to her face.

"And...I never saw her after that."

Twilight looked on, surprised.

"What do you mean?"

Another sigh from the Princess.

"The next day...she was gone. Nopony had seen her leave, and she'd taken her belongings with her. I thought to send somepony after her, to find her...but I knew she would not return even if I did."

Slowly, the Princess got up, moving over to the window while Twilight watched, continuing to stare at that sunset.

"For a time...Sunset was closer to me than anypony else. She was more than just a student...she was closer to a daughter."

A wince passed through her.

"And she...she thought of me as a Mother."

Twilight, never saying a word, watched as her beloved teacher turned, looking her right in the eye as tears began to fall down her cheek.

"But after all that...the last time we were together...that talented...innocent filly...looked at that Mother..."

There was a sharp intake of breath, and then, she finished.

"...and knew...that she had been betrayed by her."