Blasting Cadance had been a mistake. Sunset realized that almost as soon as she left the room.

A lover's spat could have been explained away. A heated verbal fight, full of bluster and big words - that wasn't unusual. Having it spill over into rash statements about the Princess would be difficult to defuse but she was a teenager and teenagers say dumb things in the heat of the moment. Yes, it would have been a major setback but it probably wouldn't have been catastrophic. But assaulting a princess and leaving her stunned on the floor? That was treason. High crime. Life in a cell, if she was lucky. Sunset couldn't talk her way out of that.

She had a few hours at most before either somepony discovered Cadance or the alicorn recovered enough to leave the room and find help. It might not even be an hour - Sunset had obviously never tested how long she could put an alicorn down for. Once that happened, the entire Palace - the nation - would be hunting for her.

Which meant Sunset had to find the information she needed and run.

There was only one place left that could potentially hold the secrets of becoming an alicorn: the infamous Restricted Section.

That library sub-basement was home to who knew what artifacts and ancient scraps of knowledge. It was warded more heavily than any other location in the Palace - even more than the Princess' own chambers - but Sunset had cracked the wards a year before. She'd never had the courage to actually break in, however, as the consequences for being caught were unthinkable. But given how dire the situation already was, well. They could only imprison her once.

Centuries-old spells cast by the greatest wizards of their generations shredded under Sunset's magic. It wasn't graceful and it wasn't clean - that would take time she didn't have. But no amount of careful arcane disassembly would stop what was coming so she didn't bother to be quiet. The wards fell apart like paper in rain, doubtlessly setting off dozens of alarms.

The path cleared, Sunset Shimmer marched into the most secret vault in Equestria. She had been here once before: a guided tour by Princess Celestia. It was here that Sunset had seen the crystal mirror that had changed her life. It still sat in the same place, off to the side as if it were unimportant and unremarkable. A little part of Sunset wanted to go look again and see if it still showed the same destiny - but she didn't have the time to spare.

It was fortunate, at least, that Sunset knew more or less what she was looking for. Not precisely but she would recognize it when she saw it. Since Celestia refused to make Sunset an alicorn and Cadance wouldn't tell her how it was done, that only left one source for Sunset to reference.

Only deeply ingrained habit prevented Sunset from simply hurling books off the shelves as she searched, her magic instinctively stacking them on the floor as they were removed. Spellbooks bound in leather that whispered forbidden horrors. Essays written by mad ponies that told of things hidden in the folds of reality. Books filled with spells that could violate every aspect of a pony in body, mind, and soul. All were knowledge any power-hungry mage would kill for and Sunset Shimmer cast them aside without caring because they couldn't provide what she wanted.

Then she pulled the book she was looking for. Reeking of preservation and security spells greater than any other item in the room, the innocuous midnight black tome held only a single clue that told Sunset it was what she was after: a silvery-white crescent moon emblazoned in the corner of the front cover.

It was the right book. But she had to be sure. There would be no coming back after she left. So Sunset cracked it open to read a random page.

Platinum came to Us today. She thought she was being subtle but We knew her purpose. She spoke first to Celestia and then to Us, probing to find what place her daughter would have in Equestria after she was gone. Her advanced age has made her legacy important to her and We could see that she was willing to strike any bargain to preserve her power. Celestia, We are told, swore an oath to never revoke the House of Platinum's title but such words do not bind Us. Still, Platinum has served as a reasonable companion these many years and despite the temptation to exact a heavy toll, We swore upon Our Moon that her house and her lineage would be respected as long as We lived.

This was it. The writings of Nightmare Moon herself. Somewhere contained within had to be the secret that made her as strong as Princess Celestia. And with it, Sunset would finally reach the destiny that had been so long denied.

Just as she made to stuff it into her saddle bag, a bright light blinded Sunset. She squinted at it - and for a moment could only make out the vague shape of a massive pony in the doorway. Her eyes adjusted and that resolved into three: Princess Celestia herself with two guards flanking her.

Everypony froze.

Sunset - unsure of how she could possibly escape.

Celestia - stunned at finding her student breaking in to her vaults.

And the guards - waiting for commands from their Princess on what to do with the unexpected intruder.

Two seconds ticked by, and then Celestia's brain registered what Sunset was holding.

Never before had Sunset seen the Princess look so gravely hurt as she did in that moment.

Before her face was overwhelmed by rage.

"you dare?"

Sunset took an involuntary step backwards as incandescent flames licked up Celestia's horn. "I--"

"YOU DARE?!" The second word boomed with all the force the Royal Canterlot Voice could muster, echoing off the walls with deafening volume and shaking the foundation stones of Canterlot itself.

Sunset's ears went flat and her eyes pinched shut under the furious force.

"After I trusted you!" Celestia's wings swept wide, threatening to topple every unfathomably precious artifact around her. "I spoke to you of things I have never shared before - gave you knowledge no other pony has! And you dare try to use it against me?!"

"I--"

"Betrayer!" The flames cascaded across Celestia's wings now. "Monstrous little--"

And then Sunset's own volcanic anger overwhelmed her shock - and she shouted back. "You call me betrayer, when everything you've said to me is a lie?!" Her head dipped, horn leveling at Celestia. The guards braced, ready to (pointlessly) leap to their Princess' defense. "Every day you made promises to me you never intended to keep! You called me student but withheld your knowledge. You knew my destiny and blocked it at every turn. Then you dangle a path in front of me not once but twice? First by flaunting Cadance then by telling me about the Nightmare - and you expected I would sit passively and do nothing?" Her hoof pawed the ground as if preparing to charge, even though the gesture seemed feeble compared to Celestia's still magmatic rage. "If you're so determined to deny me what I deserve, then what choice do I have? What can I do but use the secrets of the Nightmare to achieve my destiny?"

She panted for breath, braced for a fight. Celestia didn't give her one.

Fiery rage banked - it did not go away, but Celestia's anger resolved into a steel-hard glare. "I deny you more than that, Sunset Shimmer." The Princess drew herself higher, towering over the filly. "I have tolerated your insolence for too long in the hopes that you would learn your lesson. You have not. These selfish schemes have hurt so many of my little ponies and it is beyond time I put a stop to them." Despite the flames that still flickered between the feathers of her wide-spread wings, Celestia's voice had settled to an eerie, icy calm. "I expel you as my student. I banish you from Canterlot. The guards will escort you to gather your things and you will be on a train leaving within the hour. If you ever return I will personally imprison you within stone. Never curse me with your presence again. Now be gone, lest I revoke what little mercy I have given you."

Defiant to the last, Sunset snarled. "This is the biggest mistake you'll make in your entire life."

As the guards escorted Sunset away, Celestia's rage abated and her head drooped with weariness deeper than her bones. "One of many."

She had to find a way out. Once Sunset's anger subsided enough for her to think, she realized that she had to escape. The guards were escorting her back to her room - where they would find an unconscious Cadance. In Celestia's current mood there was no telling what assault on an alicorn would add to Sunset's punishment. Petrification was most likely, but Sunset couldn't rule out that Celestia might just plain kill her. (The sight of the flames flickering across Celestia's body would haunt Sunset's dreams for years.)

But escape from the guards was... well, maybe she could probably teleport away from them. But they would raise the alarm in seconds if she did and losing them would only delay things briefly. They would lock down the trains and there were only two gates out of Canterlot's walls, both of which would put her on long, winding paths down the side of the mountain. She needed a head start - but the guards were watching her like griffons.

They wouldn't even let her slow walk back. Every time Sunset tried to slow down to buy time, the earth pony on her right would give her a shove. There would be no sympathy from them.

All too soon they reached Sunset's room. One guard went to open the door while the other stood over her, watching.

Sunset weighed the chances she could charge and cast a teleport before he could grab her or hit her horn to disrupt it. (The odds were very, very bad.)

The door opened and the guard gasped. "Princess Cadance!" His companion turned.

Sunset froze as he spun, unsure if she was saved or doomed. From where she was, she could see Cadance still laying in that same place. Most of her was obscured by the door but the horizontal hooves and the splayed mane showed that the youngest alicorn had barely twitched.

The first guard knelled beside his princess, checking her condition.

The second guard saw the scene before him... and abandoned Sunset Shimmer to rush to the side of the downed princess.

Neither guard noticed when Sunset disappeared in a flash of light.

She should have re-appeared on the outskirts of Canterlot or at the train station. A sensible pony would have. But she'd come this far and Sunset Shimmer was never one to give up, even when it was the wiser thing to do. So she materialized at the edge of the teleport exclusion zone for the Restricted Section vault.

The doors were still open and a few steps later Sunset could see those left behind: two pegasus guards and a unicorn scribe, all busy reorganizing and reshelving the books Sunset had pulled down in her search.

Part of her felt bad for what came next. The rest understood she was only doing what was necessary and that she'd already burned too many bridges. She couldn't go back.

All three ponies in the room dropped to a quick series of stun spells, taking them completely by surprise and putting them down for the minute or so she needed. Stepping over their bodies, Sunset quickly searched for the moon-stamped journal - and found it gone. She silently cursed that Princess Celestia had thought ahead and for once hadn't succumbed to her hubris.

That was it, then. All was lost. The worst case scenario had come to pass and now Sunset Shimmer had nothing. Cadance had cast her out, Celestia had rejected her, the knowledge of the Nightmare was gone... there was nothing left for her but to run and hide and pray that she was never found. If she was lucky, she would die of old age somewhere remote and without Princess Celestia ever knowing where she had gone.

She should run. She knew she should run. She had to run.

The mirror was still sitting off to the side, gathering dust.

...But she also had to know.

Was her destiny unchanged? Would she look in that mirror and still see herself with wings of flame?

Unable to restrain herself, Sunset wasted precious seconds slowly walking over to the mirror. The train could be getting locked down that very moment - but the siren song of destiny pulled her closer.

She looked.

She saw... nothing. The mirror stared back at her with her own eyes. No wings. No crown. Just Sunset Shimmer.

A glimmer passed across the mirror's surface. A ripple, almost like water.

Sunset blinked.

There it was again - subtle but unmistakable.

Pulled by curiosity, Sunset reached out and touched a hoof to the mirror. The surface - rather than being cold and unyielding - parted like she was putting her hoof into a pool of water. Impossibly there was another side.

Starswirl's portal was open.

Sunset hesitated. She wanted to study this thing for weeks, but she... she had to run. She had to make the train or to a gate or somewhere before it was all locked down. Brain and heart fought for a brief second before she stepped away.

Up the hall and outside the door was a flash of teleportation. The scent of spring sunshine hit her nose. Golden horseshoes tapped rapidly on marble. Princess Celestia would turn the corner any moment.

A different kind of hesitation. Panic. No teleporting away, no other passages out of the room. There was only one way out.

The door opened. Purple eyes framed by a dawn mane met Sunset's cyan framed by fire. Both sets were wide with panic. The Princess opened her mouth to speak.

Sunset Shimmer dove through the mirror before Princess Celestia could stop her.

In the silence of her room, Princess Celestia hated herself. She had spent centuries keeping her emotions and her Duty apart and now she'd mixed them like ugly paint.

Not that she'd made the wrong choice. Celestia knew that. Preserving Equestria was paramount. Sunset Shimmer had delved into things that were too dangerous and she needed to be--

She bit the inside of her cheek. No. Sunset was a filly. Celestia knew how she had dealt with dangerous threats in the past and the mental image of doing that to the little foal who had fallen asleep with a book, cuddled up to Celestia in the great, empty bed... Her stomach churned.

But that was the problem, wasn't it? Her emotions had gotten the better of her yet again, and she had spoken in anger what should have been approached with calm. It had been the right choice - hadn't it? - because Equestria had to be protected above all else. But now that the adrenaline was gone and the flames had receded, Celestia realized that she hadn't acted with her Duty in mind. It was the right choice - but for the wrong reasons.

And perhaps it was her emotions taking over again, but she was already planning how to pull her wayward student back into the fold. One or two of the Guard would be assigned to watch over Sunset in secret, both ensuring she didn't continue down the dark path and keeping her safe in the wider cruel world. If she didn't dive further into darkness, in a few years Celestia would 'accidentally' run into her while touring the realm. And then the gravest of threats could be rescinded and Sunset returned to where she belonged.

Provided Celestia was alive, she bitterly reminded herself. The clock in her head that counted down until Nightmare Moon's return was a hard limit after all. Twelve years, five months, twenty-six days, fourteen hours, eight minutes. Then the Nightmare would return; they would fight because Duty demanded it; and without the Elements to aid her, Celestia would die. (She should write a letter forgiving and perhaps pardoning Sunset, just in case they couldn't reconcile before then. Best not to leave it hanging. Closure was important.)

A knock at the door pulled Celestia from her reverie. Rapid and heavy - a sure sign that the guard was in a panic. She took a second to finish what she had been doing before the thoughts carried her away and stored Luna's old journal in the dresser drawer beside the other keepsakes. Then the Princess recomposed herself and held her head high again. "Enter and report."

As expected, the panic was obvious. Still, the guard saluted despite his knees trembling. "Your Highness! I have two urgent messages for you!" She gave a slight wave, prompting him to continue. "The first is that upon taking Miss Shimmer to gather her things, the guards escorting her found Princess Cadance in Miss Shimmer's room, unconscious."

"What?"

The guard flinched. "She's alright! After being revived, she reported a fight between herself and Sunset Shimmer. She described it as a lover's spat that resulted in Miss Shimmer stunning Princess Cadance and running off. The... the younger Princess is unharmed aside from a headache but concerned for where Miss Shimmer has gone."

Tension Celestia hadn't realized she was feeling sapped away. That explained it all. It was a fight. Just a fight between lovers gone out of control. Sunset had merely lashed out and made a stupid, childish decision. Tempers would cool, some lesser punishments would be handed out, and all would be resolved in time. Sunset and Cadance both would have the chance to grow from this. "Thank you. It's good to know Princess Cadance is going to be fine. Where is Sunset Shimmer now? I would speak with her about this."

The guard didn't relax. "That's the other message, Your Highness. Sunset Shimmer escaped custody."

"WHAT?!"

"She teleported away from her escorts!" A cringe rather than a flinch. "While they were tending to Princess Cadance, Sunset Shimmer teleported away to an unknown location. We're combing the castle grounds for her now but we haven't located her yet."

Princess Celestia feared the worst and knew it was the truth: that Sunset had gone back for the book, determined to continue down her dark path. "I know where she's gone," she stated grimly. And then she vanished in a flash of light.

All Princess Celestia could do was stare at the still rippling surface of the mirror. She was dumbstruck - paralyzed. Sunset Shimmer was gone. Every memory she had of the filly rushed to the forefront, crashing into one another as they clambered to be seen in her mind's eye.

Around her, the slowly rousing guards and scribe were similarly stopped. Partially they were still recovering from being stunned; partially out of respect for the Princess; partially because a pony had just walked through a mirror. The guard on the left - Steel Sentry - glanced at his partner, head swimming. "Should we... go after her?"

Before the other guard could respond, Princess Celestia snapped her wings up to block them. "No! ...No." Her mind raced, pulling up centuries old memories and trying to run calculations. "That portal is only open for a short time. If you go through..." Her head shook. "We never bothered to track the cycles. It was... never important. Pass through and you may have days before it closes or you may have seconds. We-- I never--" Her mouth was dry. Words were missing. She wanted to run, give chase through the mirror - but Duty loomed large as the Sun. The warning was as much for her as it was her guard companions: go through and risk being trapped for years. Leave Equestria without a leader and the Sun without a guide. Duty roared louder and locked her hooves in place.

"What's over there?" Steel looked between her and the mirror, still considering.

"Another world. One which operates on rules far different than our own." Celestia's knees trembled at the thought. "Even if you went through and had time, there's no guarantee you would be able to bring her back or even find her." Her purple eyes turned to the guard. "You have a young colt, correct?"

Steel Sentry nodded, face grim. "I do."

The Princess nodded back. "I would not have him lose his father in a futile sacrifice."

"And I don't want to tell him I let the Princess lose her daughter," came his staunch reply.

She said nothing back long enough for it to be awkward. Just as Steel was about to start apologizing for overstepping his bounds, the Princess simply... smiled at him. "If only that I were so deserving." And she left it at that. "I want guards watching this mirror at all times for the next few days, just in case we're lucky and she comes back. After that, I want it moved to a more accessible location and studied. We will know when it opens next and be prepared."

"Prepared for what, your Highness?"

It was a good question that she didn't know the answer to. "Prepared for what ever happens." And the Princess left it in their hooves to handle, as Duty demanded she push on. The world did not pause for her grief when she lost Luna, nor would it now.

Author's Note