Three royal ponies sat together on a small, artificially-flattened grassy hilltop in the Canterlot Castle Gardens. The blanket they lay upon was set with a spread of pastries and hot tea. A large curtain had been erected on one side of the garden to block the Eastern area from view. Patches of charred earth could be seen here and there along the curtain's base.
"It's nice they were able to save the South Gardens from the fire," said Princess Celestia, gesturing at the lush greenery that surrounded her. "I've missed communing with nature, and I don't feel like leaving the grounds just yet."
"Baby steps, dear sister," said Princess Luna. "On the bright side, landscapers across Equestria shall have stable employ for the next few years."
Princess Twilight Sparkle shifted uncomfortably where she sat. Her magic levitated the teakettle as she made several false starts toward pouring a cup of tea.
"Twilight, is everything alright?" Celestia asked gently.
Twilight sighed and set the kettle back down, then looked up to her former mentor. "I don't know. I came here to offer my friendship and support, but I kind of feel like I'm intruding," she explained. "Wouldn't you two rather be alone right now?"
"Nonsense. My sister and I have all the time in Equestria to speak in private. Your presence is most welcome this morn," said Luna.
"Luna is right. I'm pleased to have you here with us. There is much I need to share with both of you," said Celestia. She reached out with a hoof and caressed Twilight's cheek with the back of her greave, eliciting both a blush and a look of surprise from the young mare. "There aren't many ponies who can relate to the tribulations of being a princess, after all. Although, I hope you haven't been waiting in Canterlot the whole week?"
Twilight shook her head and finally poured herself a cup of tea. "No. Not really, I mean. I did stop by last week, but I wasn't allowed to see you, so I went back to Ponyville."
Princess Celestia finished a croissant. She dabbed at her lips with a silk napkin, the smile fading from her muzzle. "I assume everypony in Equestria knows by now," she said, staring glumly down into her teacup as though examining the rising steam for some hidden message. "It's been a week since the incident, and I'm still afraid to look at the tabloids."
"Our subjects are concerned and hopeful, dear Sister. They look up to us as they always do," said Princess Luna, with a dismissive wave of her hoof. "There is no shame to bear."
Twilight nodded. "I take it you haven't seen the front steps of the castle yet?" she asked.
Celestia shook her head, then gently blew across the surface of her tea. "I've asked the staff not to mention anything outside the castle walls for the time being."
"Well, the whole area is littered with flowers and cards," said Twilight. "It's a testament to how much Equestria loves you."
"Indeed. The flotsam piles high against both sides of the walkway. It remains difficult to enter and exit without your neglected adoration getting underhoof," said Luna. "We should go visit it soon, at least for show."
"That's very nice. We should," said Celestia, and her shoulders relaxed noticeably. She took a sip of tea.
"You were under a lot of pressure, you got upset, and you needed a break. It happens to everypony," said Twilight.
"I suppose nopony is perfect," said Celestia, her visage retreating to the practiced, soft smile of a diplomat.
"Not even princesses?" said Luna, rakishly raising a brow.
"Especially not princesses," stressed Celestia, and for the briefest moment, her smile lifted into a genuine grin. "So Twilight, what has Luna told you?"
"Not much," she said. "Since my friends and I helped Luna destroy the Tantabus, I already knew she'd been using it to torture herself, which was pretty crazy. No offense."
"None is taken," said Princess Luna. "It was quite mad of me to do such a thing, and I regret my actions."
"Then when Luna told you what she'd been using the Tantabus for, you became a little, um..." said Twilight, pausing to select the best word. "Agitated?" she offered, with a sheepish smile.
Princess Celestia drew in a long, cleansing breath before speaking. "Twilight, it is important that you understand the significance of Luna's use of the Tantabus on herself. That monster was created from a shard of Nightmare that lodged itself in my heart when I banished Nightmare Moon. The Tantabus had been torturing me for a millennium." She paused to take an uncharacteristically slow sip of tea.
Luna took this as her cue. "As you no doubt recall, I did not have my mantle when I returned to Equestria," she said, pulling her magical mane to the side with a hoof as it rippled and twinkled with all the beauty of the night sky. "However, within a few months I had earned it back. With my powers fully restored, I was able to enter my sister's dreams and free her from the clutches of the Tantabus."
"I see. And then you used it to torture yourself instead. I guess it really was an act of penance," said Twilight. "Wow. I'm glad we were able to help put this thing behind you."
"It's been more than a thousand years. I believe it is high time the two of us began living in the present," said Princess Celestia. She smiled at her sister, and there was a slight shine to her eyes.
Princess Luna smiled cheerfully in return, then turned to Twilight. "Princess Twilight Sparkle, you are now current on the events as I know them," said Luna. "It is up to my sister whether she wishes to divulge anything more."
"It's just Twilight, please," said Twilight, and Luna nodded. "And you don't need to talk about anything you don't want to," she directed to Celestia.
"No. This is something I should have shared with both of you, three years ago." Princess Celestia set down her teacup and looked to her friends, her face serious. "The garden has been cleared of visitors so that the words I say can be kept in strict confidence."
For a brief moment, the only sounds were birdsong on the wind, and Twilight Sparkle choking on a very improperly-timed bite of cookie.
"I retain no memories of the millennium of Nightmare Moon's banishment," revealed Princess Celestia. "The last thing I can remember is staring up at the Mare in the Moon for the first time."
Twilight took a loud minute to choke and cough her throat clear of crumbs, then looked over to Luna. The younger of the two alicorn sisters stared at the elder, her jaw hanging agape.
Luna remained dumbstruck long enough for Twilight to fully recover. "Wait. That doesn't make any sense," said Twilight, punctuating the assertion with a final, small cough. "You're telling us you can only remember the past three years? When did your memories disappear?"
"I don't know. They've been absent since Luna's return," said Celestia.
"How in Equestria have you been able to hide this from everypony?" asked Twilight. She took a quick gulp of tea to quell her irritated throat.
"With great difficulty and planning, Twilight. Until this moment, only my personal secretary Raven and I have known the truth," said Celestia. "Apparently, every sixteen years I cast the Lethe spell to wipe my memories of the previous sixteen-year period. It's the only way I've been able to manage the pain of the Tantabus during my sister's absence."
Luna finally closed her jaw, blinked a few times, and entered the conversation. "Naturally, I know the mythology of Lethe, the Spirit of Oblivion. However, I know of no such spell," said Luna.
"Technically, neither do I," said Celestia. "According to my notes, I cast the spell on the Tree of Harmony a millennium ago. All I need to do now is perform a simple ritual beside the Tree once every sixteen years. Doing so at the proper date and time reactivates the spell."
"You leave notes for yourself? Right, I guess you'd have to," said Twilight.
"In extensive detail," added Celestia, pausing to lift the cup back to her lips and take a sip of tea. "The two weeks leading up to every sixteenth Summer Sun Celebration are spent ensuring that my diplomatic and personal records are as complete as possible before I 'renew myself', so to speak. Then I go to the tree, perform the ritual, and return to Canterlot on the eve of the Celebration. Fortunately, the Summer Sun Celebration's ceremonies are so simple I need not memorize anything in advance."
"What are your earliest memories of the current era, if I may inquire?" asked Luna, before biting into a scone.
"I remember waking in front of the Tree of Harmony in the Everfree Forest. To my surprise, the Elements were missing from it, and the castle ruins were nearly unrecognizable," said Celestia. "A small scroll lay beside me, which read, 'Return to Canterlot immediately to prepare for this year's Summer Sun Celebration'."
"Wait..." said Twilight, as the gears in her head began to turn. "That's why you sent me that letter rebuffing my warning about Nightmare Moon's return, isn't it? You really didn't remember the legend!"
"You are correct, Twilight. Your scroll arrived while I was foraging berries, which made for a simple ink," said Celestia. "I was able to use the back of the scroll to return your message. From my point of view, I had just met and dealt with Nightmare Moon mere minutes ago. Your message and my environs together suggested that I had been transported forward through time. It was daylight, after all, and I did not remember raising the Sun. But daylight also implied that Nightmare Moon had not yet returned."
"Hmm. Well, I guess my message told you who I was. But you responded with detailed instructions," said Twilight.
"As an experienced diplomat, I am aware of the intrinsic value of information," said Celestia. "I was acting under the assumption that something bad had happened, since the Elements were neither on my pony nor in the Tree. Your letter could have been a ruse by one of Equestria's enemies—we had plenty of them during that era of our nation's history. I didn't want to tip my hoof that I didn't fully understand the message, so I responded as quickly as possible with general suggestions."
"The clever mare as always," said Luna, between cookies. "By stating general facts, you would not reveal the full extent of your ignorance. Yet, your recipient would be likely to correct the missing portions by supplying more information to you."
Twilight scrunched her muzzle up in thought. "I'm still confused. In the letter, you told me to forget the legend about Nightmare Moon's return. You said you had a new assignment for me, which was supervising the Celebration in Ponyville," she said. "Also, you told me I needed to make friends."
"I didn't know what the Celebration was, but assigning you to supervise it was the best bluff I had available. As for the advice? Everypony needs friends, Twilight. It was clear from the tone of your letter that you were a highly self-reliant young pony. If this truly were a letter from my personal student, suggesting they consult other ponies would surely help them to solve whatever problem they were facing," said Celestia. "But if, as I suspected, the message were some manner of trick, a rapid response would hopefully delay my antagonists or at least convince them to give me more information. Either way, I needed time to locate the missing Elements before the prophecy you mentioned came to pass."
Princess Luna swallowed the remains of a biscotti before speaking. "One issue yet remains. Ponyville did not exist one thousand years ago," she pointed out.
"I didn't mention Ponyville. I just asked Twilight to supervise preparations for the Celebration in 'this year's location'. I assumed it was happening in Canterlot," said Celestia. "I later learned Raven had already asked to have the Ponyville Library vacated for your stay."
Twilight Sparkle rolled her eyes. "Ugh, Spike must have ad-libbed the word 'Ponyville' while reading me your letter. I hate it when he does that! See the sorts of confusion even the tiniest lack of attention to detail can cause?"
Luna chuckled briefly at Twilight, then turned her attention back to her sister. "It does seem odd you would not notify Twilight prior to your 'cleansing' that she would be supervising the Celebration," she said.
"That's the only part I don't understand. My notes say nothing about plans for Twilight to supervise the Celebration," said Celestia. "There isn't any mention of the prophecy in my notes, either. Some of the records from the earlier cycles have missing pages. I suspect at some point I destroyed that information, but I can't imagine why."
"So it's just random coincidence that Raven planned for me to supervise the Celebration at the same time that you blindly gave me the assignment?" asked Twilight, her eyes narrow with skepticism.
Princess Celestia shrugged. "The Tree of Harmony has a storied history of mediating destiny," she offered. "The force that drew you to Ponyville is undoubtedly the same one which fated you to meet your friends when you each received your cutie marks from the first Sonic Rainboom."
Luna's face soured. "This also explains what you were doing in the old castle ruins when Nightmare Moon found you there, Sister. I have vague memories of being that monster. They are not unlike memories of a true nightmare," said Luna, and she shivered at the thought. "Even what the Tantabus did to me pales in comparison."
Twilight took a sip of tea, then set down her cup. She knitted her brow tightly and looked up to Celestia. "Why did you keep this a secret from us for so long?" she asked. "Once you were free of the Tantabus, there was no need to hide any of this, was there?"
Princess Celestia closed her eyes and paused for a few seconds. Her lower lip twitched slightly. "Guilt," she finally said, as she reopened her eyes. "I feel guilty for losing my memories of you, Twilight. I didn't want to admit it to you, and I didn't want you to find out from anypony else. I knew I'd need to tell you eventually, but I kept putting it off. After all, it seemed a modest burden compared to what the Tantabus had been doing to me."
"Ah. So this secret was yet another straw upon the donkey's back," Luna postulated.
Celestia nodded. "I think so, yes. Apparently, keeping this to myself weighed on my soul more heavily than I'd imagined it could. Recent events seem to illustrate that even alicorns have our limits."
The look on Twilight's face slowly drifted from confusion to horror. "Oh my Stars... You don't remember me being your student," she realized. Twilight turned her face away from Celestia and toward Canterlot Castle. Her voice quivered in her throat as she spoke. "You know... For the past three years, you seemed so distant. At first I thought it was some new kind of test, but after about a year... I guess I decided the closeness I felt with you growing up wasn't real."
"I'm so sorry," said Celestia, as she reached out with a hoof to turn Twilight's chin back toward her. The younger mare looked like she was holding back tears, and would not meet Celestia's gaze. "Out of everything I've endured, losing my memories of you pains me most. It's like I erased your childhood and the all of the lessons we learned together. You deserve better."
Twilight grimaced. "I guess it's not your fault," she whispered, looking up into Celestia's eyes for a fraction of a second before lowering them again.
"Isn't it? I wasn't strong enough to avoid casting the spell, even though I knew it would erase the details of our relationship from my mind," said Celestia, and then her voice grew resolute. "Twilight, I want you to know that what I am about to tell you is not something I say lightly. My notes from the previous cycle speak of you in the highest possible regard. I've never known a pony with your raw talent and drive to succeed. Even though I didn't appear to know about the prophecy of my sister's return, I speculated you would someday find a way to bring her home. I recorded dozens of detailed anecdotes from my experiences with you. I was very fond of you, Twilight Sparkle. I cherished the moments we spent together."
Twilight nodded and sniffled, then wiped at her watery eyes with a hoof.
Luna cleared her throat noisily. "Forgive me for steering the conversation at hoof away from the maudlin and toward cold logic, but I believe steps should be taken at once to restore your memories, Sister," said Luna.
"I agree, of course. I've already tried by myself, but with limited success," said Celestia. "Immediately upon reviewing my notes from the previous cycle, I told Raven of my intentions to recover my memories. She was adamant that it could be disastrous, and as the sole link to my previous self, I followed her advice. However, once you freed me from the Tantabus, I made an attempt to recover my memories without telling Raven. Curiously, the spell was only able to restore my memory of the first four years following Nightmare Moon's banishment."
"What did you learn?" asked Twilight, as curiosity quickly edged the sadness out of her voice.
"Very little. I remember the initiation of the Summer Sun Celebrations, and the initial renovations to Canterlot. I suffered the Tantabus nightly, despite all magical and medical attempts at aid. So I slipped into a deep depression," said Celestia. "Star Swirl the Bearded made his prophecy that Luna would return in a thousand years, but it did nothing to cheer my mood. I couldn't handle the thought of waiting that long, as each day seemed longer than the last. Shortly prior to the fourth Summer Sun Celebration was the first time I cast the Lethe spell."
Twilight looked up into the air and her eyes darted back and forth as though she were reading an invisible book. "No, that doesn't fit," she said. "Even if there were a Year Zero..."
"There was a 0 Anno Solequus," interjected Luna. "The first Celebration was held one year after our defeat, and due to superstition, scholars insisted on renaming that initial year."
"...yes, I know that," said Twilight, rolling her eyes. "All I mean is that it doesn't matter because we're counting backwards from 1000. Since 1000 modulo 16 is 8, either you didn't reliably cast the spell every sixteen years like clockwork, or the first casting should have occurred one day prior to the Celebration of 8 A.S. That's at least four years later."
"Your math is correct, of course. My first casting of Lethe in 4 A.S. seems to have been a trial run with the spell. I didn't use the Tree of Harmony to cast it then," said Celestia. "The spell was supposed to suppress enough memory to remove the traumatic events from my mind. This removed all of my memories back to the point where the Tantabus infected me. It did not, however, remove the Tantabus."
"The spell you describe would not be flexible enough to use repeatedly for an entire millennium, even if the Tree of Harmony were used as a focus," said Luna. "You must have modified it significantly, or used a different spell entirely."
Princess Celestia slowly bobbed her head in agreement, the glowing pastel hues of her mane flowing gently behind her despite a lull in the breeze. "That is true. Unfortunately, I don't have a transcription of either spell in my notes. All I have is a short letter written in 8 A.S. which describes my plan to recast the spell every sixteen years. It provides the ritual used to activate the spell, but that's all."
Twilight pursed her lips in thought, then smiled brightly. "I just realized something. If the two of us cast your memory restoration spell in conjunction with you, we should be able to recover the rest of your memories!"
"Yes. I was thinking along the same lines, hence my suggestion to proceed," said Luna. "In point of fact, even if the Tree of Harmony were involved, only two alicorns should be necessary to overcome the resistance of a spell cast by but one alicorn."
Celestia nodded. "That's what I want to try today," she said, then turned back toward her former student. "Twilight, even though my memories of you are what I miss most, I feel it would be best to perform the casting privately with Luna. Being awash in a millennium of experiences will no doubt be overwhelming, and I know Luna far better than anypony else."
"I understand," said Twilight, and she clenched her jaw for a moment. "You want to do this the right way, and it will be safer if it's just you and Luna present. You'll come and tell me as soon as you know, right?"
"Of course," said Luna. "We should perform the casting in Celestia's bedchambers, and you may wait elsewhere in the castle."
"Excellent," said Celestia. "I very much hope it works. I'm looking forward to getting to know you better, my faithful student."
Twilight smiled wanly. "Well, even if it doesn't work, I think I've grown so much over the past three years that you probably know me even better now than you did back then," she admitted. "It would be a lot worse to lose the past few years. Even today's experiences have been valuable for me."
Celestia nodded, and placed a warm wing around Twilight. "I feel the same way. I owe you a persistent memory, Twilight, and I look forward to the years yet to come. I have no doubt they will be even brighter still."
Luna stood up. "I see no reason to dawdle further. Let us commence at once."
Twilight and Celestia exchanged a quick glance, and all three headed back to the castle.