Facing a serious decision between two sides, anypony would tell you they examine every factor carefully and impartially before making their choice. But the truth is that beyond any sort of critical thinking, they have a subconscious bias in one direction on the other, simply based on word association. A single phrase, maybe an image, so strongly connected to a pony or cause that they cannot mentally divorce the two, and that phrase or image will forever influence their view of them. This is why first impressions are so important; they usually decide that association in the minds of both parties.
Some ponies associated Princess Celestia with "wisdom" or "experience", given she had been ruling Equestria since well before they were born. For Nightmare Moon, they had "villain" or even "terrorist".
Conversely, some ponies associated that name with "utopia".
There were a few tense, silent moments, before one unicorn stepped out and crossed the battle line. She turned to stand beside the Shadowbolts, then told the others calmly, "No more tyranny."
Another followed, this one a pegasus somewhat older than his predecessor. He said nothing, but smiled at Nightshade, whose eyes had been locked on him since he stepped out of the crowd. She smiled back and nodded, as her eye seemed to shimmer with moisture. But that moment was all she allowed herself.
Now the crowd was starting to shift, as more and more ponies came forward to join the New Lunar Republic. Celestia (and those still loyal to her) could only watch in shock as friends, even family stepped across the line, to be warmly received by their Princess of the Night. In the end, the opposing camps stood equal, the population of Canterlot split in two.
"No..." Celestia had expected some dissidents, but not half of Canterlot to turn on her. The sight was wounding, and in response she lashed out in the only way she could think of. "Guards, arrest them!" But no sooner had she finished the phrase, she saw some of her own guards walking across the no-pony's-land. Each approached Nightmare Moon, then stood firmly, pounding their right hoof to their chest in the salute of the Lunar Guard. She returned the gesture with pride.
"Betrayed... by my own guards?"
"They were never yours in the first place, sister. They are the surviving Lunar Guard, hiding in plain sight until this day. Even more are scattered across Equestria, sharing my vision in every city, waiting for my call."
"It does not matter. However many ponies you may draw to your side, I will end this rebellion now!"
"Oh yes? To coin the phrase, you and what army?" It was a point well made; between Nightmare Moon's original insurrection, the changeling invasion and this recent division, Canterlot's Royal Guard was beginning to look a small force indeed.
"Her and me!" This voice was an unexpected one; Shining Armor had made his way to the frontline and stood beside his sister, weakened but resolute. "I, and every pony standing with me, will always defend Equestria against traitors like you." Emboldened, the Elements stood firmly beside him.
"Haven't we been though this already?" Rainbow Dash wondered aloud. "You couldn't beat us then, and you can't beat us now!" But what she couldn't see was that far from them, in the ruins of Canterlot Tower, a bolt of light punched out of the debris. It arced over the city like a firework, before exploding into six distinctly colored fragments, which rained to the street below.
"The Elements of Harmony!" Twilight cried in surprise. The Element of Magic had landed on her head, and each of her friends had been bestowed with their respective necklaces. But she looked back to see Rainbow Dash, while equally awestruck, no longer bore the Element of Loyalty. Instead, that pendant had made its way to her brother's neck, to his great surprise. "The Element of Loyalty has chosen a new bearer!" Celestia observed.
Rainbow Dash looked to Nightmare Moon for guidance: all the ponies in the world couldn't give them a fighting chance against the Elements. "I had hoped this would not come to pass," she told her apprentice, "but I did plan for the possibility. This battle is not yet lost, but the war will not be won today."
"Rainbow Dash!" Twilight called out, "Look! The Elements themselves declare your betrayal!" She was launching into what Dash had once mockingly referred to as a "Twilight Sparkle speech". "Turn away from the path of darkness..." Blah blah, friendship is magic, yadda yadda... But as the crowd was focused on Twilight's speech, Nightmare Moon quickly began teleporting her followers away one by one, each pony disappearing in a flash of blue light.
"I need you to lead them, Nightshade. Use the castle as shelter and start rebuilding it. We'll be there as soon as possible," she whispered to Nightshade, now that the last Republican was safely gone.
"Yes, Princess Luna."
Seeing that her student's oratory was falling on deaf ears, Celestia stepped forward and held her back with one hoof. "Nightmare Moon! Shadowbolts! I find you guilty of crimes without number and villainy without end. The Elements shall mete out justice as they see fit." She stepped back, and each Element began to glow and crackle with power.
"Charger, it's time," Nightmare Moon replied calmly. He nodded back without emotion, reached to his belt, and quickly triggered his last detonator. Just as the newly-formed Elements of Harmony began to charge their final sanction, the ground leapt up beneath their feet, and the entire city shuddered.
"Coal Dust! Hey, Coal Dust, come quick!"
Lucky Strike was a mare that enjoyed routine. She thrived on the predictability of life as an undercity inspector, even if the pay was lousy. The entire complex was very sturdy and well-made (even if it didn't always look that way), so when she and her partner went down on their four-day 'tours', she could tell almost like clockwork how it would go. They would tour one section of the undercity, being sure every essential this, that, and the other thing were properly in place (they always were), then spend the night in one of the rest shelters. The next day, they would go to a new section and repeat the process until their tour was complete.
The body of a missing inspector kind of got in the way of that routine.
"Sweet Celestia..." Coal Dust, a former mine worker who was very at-home this deep below the surface (in a way Lucky Strike wasn't entirely comfortable with), was close behind. He trained his headlamp on the body, along with the large section of metal catwalk that had pinned it to the floor. "You think...?" They both looked up, into a darkness their lights could barely penetrate, and could barely make out a gantryway that did indeed have a large section missing.
"An accident?" Lucky speculated, already trying to flip the piece of metal off the dead inspector. "I remember he was assigned to the upper sections... maybe it gave way beneath him?"
"Then why did it end up on top of him?" Coal pulled the body clear, allowing Lucky to drop the metal sheet, then examined the inspector. "And look at these wounds. That's shrapnel... and his throat's been slit! He was murdered!"
"Nonononononono this is not happening!" But before Lucky got the chance to hyperventilate, a massive blast of fire and light went off overhead. Even as it passed the floor beneath her hooves continued to shake, throwing her off-balance. But she was more concerned with the clank and clatter of loose metal above, some of it already tumbling down to their level.
"Lucky, run!" Suddenly, Coal Dust shoved her away from him, but when she looked back she could only see a massive steel girder where her partner must've been.
Various ponies screamed in fright as the ground itself began to shift, becoming an ever-steeper grade toward the city's outer edge. It settled maybe a second later, but that did not stop the panic. Several carriages, abandoned in the invasion, began to roll, picking up speed as they careened down empty streets before ramming and toppling over the protective fence. Startled, their concentration broken, the Elements lost power.
"What have you done, Luna?!" Celestia accused.
"High-yield explosives stolen from your armory, placed strategically around the undercity," replied Charger. He held up the detonator. "Sort of a last-resort gambit; in case of Elements of Harmony, press here."
"But that would..." The consequences dawned on Twilight. "No! You couldn't possibly!"
"Give me blueprints, nitro and time, I can bring anything down," he bragged. "Including this city," Nightmare Moon added threateningly, before teleporting Charger away with the other Shadowbolts, Rainbow Dash being the one exception. Almost in response, the city began to shift again, thought not half as drastically as it had the first time. But the entire loyalist camp was shaken, still trying to wrap their heads around destruction of such magnitude. Even the Princess herself was finding it difficult to maintain composure.
"Looks like this is the fall of Canterlot!" Rainbow Dash taunted from above. "So who means more to you, Sunbutt? Us... or them?" Celestia turned to see the hundreds of frightened ponies in her faction, looking to her for guidance (at least, those who had not already fled in panic). Quick-thinking Twilight stepped toward her, hopeful against hope. "Princess, if you want to lead the evacuation, we can..."
"No." Looking up, the unicorn saw blood in her teacher's eyes, narrowed scornfully, which was unprecedented in the unicorn's memory. On balance, it frightened her even more than the situation itself. "My mercy, my indecision, my weakness allowed this to happen. I must put an end to this war, permanently."
"But Princess..."
A spark of anger shot through Celestia, unrestrainable. "Go, Twilight! Get these ponies to the caves, then round up anypony else still in the city!"
Twilight knew there was no point in arguing, nor the time; she looked at the others, and they spread out in front of the crowd. She then teleported to the other side, and together the team began herding the crowd in the direction of Mount Equus. As they left, Celestia turned back to Nightmare Moon, who had been quietly speaking with her apprentice.
"Hear me, Nightmare Moon!" she screamed. "You and your followers have been a blight upon Equestria, and caused more suffering and death than has been seen since the Time of Discord! I was a fool not to destroy you at the first opportunity. Now I correct that mistake!"
"Promises, promises..." Dash commented wryly, pacing through the air above her mentor. Though she refused to reply directly, Celestia's temper only rose.
"Canterlot may fall today, but never again shall you terrorize Equestria!" And with that she charged, cloaked in a swirling blaze of solar energy. Nightmare Moon, who had been otherwise expressionless, allowed herself a momentary smirk before collapsing into mist and dissipating. Rainbow Dash took to the sky, easily avoiding the attack. Celestia pounded the empty street, then came to a stop and redirected herself upward in pursuit. But it was all too easy for Dash to dart away, weaving between buildings and in and out of sight.
"Don't waste time on my apprentice, sister! I'm the one you want!" Nightmare Moon called out from below, stealing Celestia's attention; she turned about and loosed a volley of magical bolts, which exploded brilliantly against their target's energy shield. When the dust cleared, Nightmare Moon had vanished again.
"Yoohoo! Over here!" This catcall and a quick telekinetic grab were all that saved the Princess from an incoming dagger, as Rainbow Dash took center stage in the fight once more. Celestia hurled blast after fiery blast of magic, but her opponent was literally flying circles around her, and was just too fast to hit. With a scream of rage, she loosed a radial burst of power which knocked Dash away and gave her a window of opportunity. But as she moved in to attack, a force from beneath rammed into her, and she was flipped upside-down as Nightmare Moon drove her toward the concrete.
With a blast of magic, Twilight Sparkle blew open the front gate of the palace, beckoning the crowd to enter. Even with their shared urgency, a group of that size could only move so quickly. Fluttershy even had to dive in on occasion to rescue a stumbling pony from being trampled. They ran up the staircase only to be bottlenecked at the main doors. And those who made it inside found themselves at a standstill: Twilight had only a vague idea of where the entrance to the cave system was, definitely not enough to lead the herd by. But soon a young mare approached, who Twilight recognized as Celestia's adviser, Fortitude.
"What's going on?"
"The undercity is about to collapse, we need to get these ponies into the caves!"
"And the Princess?"
"She's fighting Nightmare Moon and Rainbow Dash. But I don't know how to get to the access tunnel, can you help us?"
"I know the way, come on!"
Fortitude took the lead, guiding the crowd downstairs and into the palace sub-basement. Weaving through a few corridors, she forced open a heavy wooden door, revealing a rocky, torch-lit tunnel. Twilight and the ponies right behind her kept moving as she stayed back to hold one of the doors open; Shining Armor grabbed the other. They kept waving ponies through, urging them onward, until they met Pinkie Pie and Rarity at the tail of the group.
"What happened to Applejack? And Fluttershy?" Shining Armor asked.
"They went to survey the rest of the city, round up anypony else they could find," Rarity replied. "I left a trail fo-" She was cut off by an unearthly shriek from down the tunnel, followed by cries of panic and fear. Exchanging worried looks, the four of them started to push through the crowd, Shining Armor in the lead.
The Princess of the Night pulled up at the last moment, hurling Celestia into the pavement with all the force she could muster. But she was back on her hooves more quickly than expected, surprising Nightmare Moon with a concentrated beam of pure heat, which shunted her out of the sky. She barely had time to throw up a shield before the same beam washed over her again, this time exponentially stronger.
"No mercy from the Elements this time, Nightmare Moon! For your treason, I sentence you to death!" Celestia screamed, unrelenting in her attack.
"The judgement of a tyrant means nothing to me!"
But Celestia was so focused on keeping Nightmare Moon pinned down, she barely had a chance to notice Rainbow Dash's incoming dive-bomb. In fact, by the time she could redirect her beam, Dash already had her sword drawn, cutting right through it as she charged. The blinding glare meant Celestia never saw the blade coming, until it made contact with her horn, disorienting her. Dash followed up with a spinning kick, knocking her foe back and giving her time to disappear back into the skyline. But Celestia could still see Nightmare Moon as she recovered, and she hastily fired off another burst of magic; it only whiffed through vapor before punching into the side of a building further down the road. The entire city shook as the structure came down.
"Tsk tsk, Celestia. And I thought I was supposed to be the one destroying Canterlot."
The voice came from nowhere in particular, but Celestia fired another blast where she thought the source was. "Show yourself!"
The only answer was the growing crumble of concrete, which gave Celestia enough warning to dodge away as another building toppled onto the street. The dust cloud it kicked up cast an eerie twilight on the area, darkening what sunlight could get through the Canterlot skyline. Aggression tempered by uncertainty, she circled in place, hooves spread wide in a tense defensive posture. She knew the next attack could come from any side, and still shooting at anything that moved, she told herself the next one would definitely be her hated foe.
"Princess Cadence! Please stop!" Twilight cried, blocking another violet bolt of energy.
"Nonono, you're not her! You only look like her!" the disheveled alicorn raved in reply. "Your tricks don't work on me, you Faust-damned BUGS!" As Shining Armor and the others reached the front of the crowd, they found Twilight holding up a force-field to separate them all from the Princess of Love.
"Cadence! Twilight, what's happening?"
"This is the real Cadence, Shiny; Queen Chrysalis must've trapped her down here in order to take her place!"
"Her! Where is she? Where is your queen, vermin?! She doesn't get to just steal my life and leave me here to rot! Do you hear me, drones?!"
"She thinks we're all changeling imposters," Fortitude observed.
"But we're not! Cadence, listen to me, it's me, Twilight! Don't you remember me?"
"No, no, that's just what they'd say, that's just what they'd want me to think! But I'm not that easy! You don't fool me, not-Twilight!"
"Cadence, please! It's me!" Shining Armor pressed himself against Twilight's shield, trying to be as close to his lover as possible. "You're the mare I love, Cadence, please, please come back to me."
Cadence stared for a little while, dumbstruck; she almost seemed calm. But that was before she lashed out with a furious barrage of haphazard magical attacks. "NO! How dare you use him against me! Nothing is beneath you hive-spawned bastards, is it? Well you'll have to go lower than that, do you hear me, because you'll never break me! Never!"
Shining Armor turned to his sister with a broken look in his eye. "There's no telling how long she's been down here, or just what Chrysalis has done to her," she offered, hoping knowledge would be some consolation.
"What can we do for her?"
Twilight thought for a moment, as Cadence continued to pound against the indigo force-field. "...We need to keep moving. For now the best I can think of is a sleep spell. Once Princess Celestia has dealt with Nightmare Moon, we can all work on helping Cadence recover."
All Shining Armor could do was look at his hooves, crippled by a blend of heartbreak and frustrating helplessness. "Do it." In the meantime, Cadence had relented to catch her breath. Twilight took this opportunity to drop the shield and seize her with magic.
"It will take time, Cadence is very powerful and very upset..."
But before Twilight could even get started, another pony charged out of the crowd, turned and bucked the Princess of Love clean in the jaw, knocking her out. "That's an earth pony sleep spell for ya," Applejack commented dryly. Shining Armor had to restrain himself from a kneejerk reaction to somepony attacking his fiance, knowing that in the end it was probably necessary.
"Applejack!" Twilight noticed with surprise. "What are... I mean did you and Fluttershy find anypony else?"
"Yeah, we caught a few stragglers, brought 'em all down here like you said. Twi, there's gotta be more of 'em out there, I just know it! We have to..." A solemn, admonishing look from Twilight cut her off.
"Canterlot could fall at any moment. It would be impossible to save everypony."
"...When I heard all the ruckus I came up front to see what was goin' on."
The cogs started turning again in Twilight's mind. She quickly drafted two stallions from the group to carry Princess Cadence, then turned to Fortitude. "If you keep going down these tunnels, what is there?"
"I'm pretty sure it lets out at the ruins of Old Canterlot."
"Perfect. I need you to lead these ponies and keep going; we'll meet you there when the battle is over."
"But what about you?"
Twilight pointed to the gem-embossed tiara she still wore. "We've got to help the Princess." She gathered the other Elements, and together they charged back through the cave system and toward the city they hoped was still there.
"Looking for me?"
Rainbow Dash seemed to materialize immediately behind Celestia, who instinctively whirled around and shot a blast of magic at her as soon as she was in sight. Dash casually batted it away with her sword.
"Do not take me for a fool, Rainbow Dash. You are nothing but a distraction, so your mistress can strike me in the back." To accentuate the point, she put up a force field around herself, covering every angle outside her line of sight. Dash only shrugged, unimpressed. "If you say so, Sunbutt."
But then an idea crossed Celestia's mind, something born of all her anger and desperation. She telekinetically seized Rainbow Dash, holding her aloft and immobile with all her magical strength. Dash struggled fiercely to break loose, but Celestia held tight, refusing to give even an inch of freedom. "Nightmare Moon!" she shouted to the empty streets. "Come out and give yourself up!"
"Or what?" the empty streets replied.
"Your apprentice dies, this time in earnest!" Canterlot shuddered once again.
"Ooh, Celestia's gotten gutsy!" Dash mocked. "Just like Twilight, big on threats but not on follow-through."
Celestia released her grip on the Lunar Apprentice's head just long enough to throw a punishing right hook into it. "Do you think I'm bluffing?" But Dash only laughed as she looked back, blood starting to trickle down her mouth. "Yeah."
Furious and frustrated, the Princess turned to Rainbow Dash's katana, slowly and forcefully prying her hoof loose from it. She picked it up, balanced it in her hoof, then leveled the point squarely at her opponent's chest. "One chance, Nightmare Moon!" The tip began to press into Rainbow Dash's flight suit, about to draw blood...
"You called?"
Nightmare Moon suddenly appeared behind Celestia, a gaunt, spectral figure.
"Give yourself up and I'll spare her life," she replied, turning around but keeping the sword keenly aimed.
"My apprentice was right, this is unlike you, sister dearest. Even I can't remember the last time you had blood on your hooves. Would you really do it? Take a young mare's life?"
"I wouldn't bet on it," Rainbow Dash enthused smugly. Though she was obviously infuriated, Celestia remained stoic. "If you want a war, war you shall have, Nightmare Moon. And this is the consequence." She brought her hoof forward, the drawback in preparation for a killing blow. But before she had the chance to follow through, her own shadow leapt up and ensnared her, thorny vines of darkness digging into her foreleg. "Ahh!"
Nightmare Moon began to cackle maniacally, before vanishing (though her laughter did not stop). "Gullible as always, sister." She rose up beside her, the shadowy tendrils that now bound Celestia completely trailing back into her ethereal blue mane. "All your sturm and fury, outmatched by a simple illusion." The dark regent forced her sister's hoof to the ground before pulling the sword free of her grip, then setting it carefully down. A shock of magic sent Celestia into spasms of pain, forcing her to release Rainbow Dash.
"All too easy!... Ooh, sorry, were you gonna say that?"
Nightmare Moon smirked knowingly in reply before turning to Celestia. She said nothing; instead, even more waves of dark magic rushed into the Princess, crackling like electricity. She screamed desperately as every part of her body was overwhelmed with excruciating pain. When the tendrils rescinded, it was all she could do to keep standing.
"....D-d-d-do your worst."
And after circling the downed monarch to stand on either side of her, that's exactly what they did.
Rainbow Dash opened up with a vengeful right uppercut, which Nightmare Moon followed with a brutal hammer-strike to the back of Celestia's head. She jerked back up instinctively, opening herself to a spinning kick to the breastplate from Dash. She tried to let off a blast of magic to defend herself, but it went wild as Nightmare Moon bucked her from the side. Meanwhile, Dash scooped up a broken piece of pipe from the street, then swung it hard into her enemy's face two times, in a forehoof-backhoof combo. She tossed the improvised weapon to her Princess, who caught it magically before striking Celestia's left foreleg. She buckled and collapsed under the pain, until an aerial stomp from Rainbow Dash drove her even deeper into the ground.
Nightmare Moon quickly set up the follow-up, lifting Celestia magically, but only long enough for Dash to fly up, loop backwards and dropkick her into a concrete wall. But she spent hardly a second on the ground, as Nightmare Moon yanked her back telekinetically on a reverse course, this time directly into an incoming punch from Rainbow Dash. Equestria's monarch skidded across the pavement before coming to a stop, lying motionless.
Walking over calmly, Dash picked up her katana, flicking it through the air several times to shake the dust off. She turned to Celestia, only to watch her vanish in a flare of violet. "We have company," Nightmare Moon observed, looking up the street. Maybe a block away, Twilight Sparkle was knelt over Celestia, the rest of the Mane 6 at her side. "...You monsters!" she screamed, but was cut off by the rising grind of rending metal. Canterlot began to dip even further, so that even to stand upright was a challenge.
"What do we do?" asked Rainbow Dash, concerned and just a bit afraid; she remembered wielding an Element of Harmony, and had no desire to be on the receiving end of its power.
"We stand our ground."
"What?!"
"Rainbow Dash, everything I have taught you, all our training and meditation, has been to prepare you for something more."
"Wait... what do you mean?"
"We have only one chance to stand against the Elements. My loyal apprentice, do you trust me?"
"...Of course, Princess Luna."
"Then calm yourself, and open your mind."
Rainbow Dash paused for a moment, wary, but then sat obediently, laying down her sword. A sphere of magical energy formed around them, as Luna slowly began to chant. "Power of night, power of flight. Power of night, power of flight. Power of night, power of flight, that stand and fight the wicked sun..."
Stepping forward, side by side, the Elements of Harmony focused themselves and began to charge their magic.
"Power of night, power of flight. Power of night, power of flight. Power of night, power of flight, defy the light and be as one." Luna repeated this creed, slowly growing in speed and intensity. Dash could feel the magic suffusing the air itself, restoring her, fueling her strength. Even inside her mind, her emotion and drive had become unrestrainable. Though she could not see it, her sword was slowly turning in place, hilt toward her like a bridge between the two of them.
Each of the Mane 6 started to glow with light, as they rose into the air and began to drift together.
The sphere had become enveloped in absolute darkness. The only sense Rainbow Dash knew was Luna's voice, still repeating the chant. "Powerofnightpowerofflightpowerofnightpowerofflight..." That is, until the magic literally surged into her body. She could tell she was changing, but any shock or fear was overwhelmed by adrenaline and new-found strength in her every muscle. But then an intense pain shot through her head, and the sheer sensory overload caused her to black out.
Six separate beams of energy combined into a single, devastating blast, the full power of the Elements of Harmony hurling toward the dark orb. Upon impact, there was a blinding explosion of intertwined light and shadow.