Alicorns possess the senses of all the tribes and more besides. Celestia felt the disturbance not through horn, wings, or hooves, but through her very essence resonating with another that had risen to a higher level of being. She beamed. She loved it when the best-case scenario played out.
Celestia poked her head out of the door of her royal chambers. "Gentlecolts," she said to the guards at either side, "I have some pressing business to attend to. See to it that nopony disturbs me until further notice." Remembering the unfortunate incident with Private Blind Obedience, she added, "Or the end of your shift, whichever comes first."
Both stallions found a way to stiffen up even more than usual. "Of course, Your Highness," they chorused.
Celestia nodded, closed the doors, and settled herself on her bed. She shut her eyes and meditated on her most fundamental self, on radiance and authority and love. Her power filled her every cell with pleasant warmth, and she vanished in a flare of purest sunlight.
Spike's letter materialized on her pillow scant seconds later.
Celestia strode through the mists of the Astral Plane. The strange stars overhead were not her sister's and conformed to no known constellation, but she was confident that she was going where she needed to be. The forces of destiny, tangible in this realm, gently but inexorably tugged her along the path they needed her to take.
Celestia's heart soared. She knew the spell would work, that Twilight would make it work. Celestia had far more faith in her student than herself, and this latest triumph showed just how justified her faith was. As soon as she saw a shape in the distance, the words she'd longed to say since Sombra's defeat came spilling out. "Congratulations, Twilight."
"Princess?"
Celestia stumbled to a halt. That wasn't right. That was about five extra voices worth of not right. Destiny pulled ever more forcefully until her legs started moving of their own accord, her mind still trying to figure out what was going on. "Are you alright?" She could see something in the mist now, definitely equinoid, definitely alone.
"That's... complicated," said the voices.
The image cleared. Celestia blinked. "So I see." At first, it seemed like see was looking at another alicorn, but the horn and wings were translucent, and they weren't the same color as the mare's body or one another. It was the mane that made matters clear for Celestia, cycling through half a dozen styles almost faster than the eye could follow. She looked at the cutie mark and confirmed that it too flickered through six different symbols.
After a few more moments of taking in the being before her, Celestia whispered, "What have I done?"
"Actually," said the composite mare, "this one was my, uh, Twilight's fault. Probably. Mostly. You did give her the journal."
Celestia latched onto the last part. "Star Swirl's spell did this?"
"Well, after... Twilight read through all of the journal, she was able to approximate Star Swirl's thought process, and she determined that the Elements of Harmony were essential to the spell's intended purpose. So, she called the rest of us, we all put on our respective Elements, Twilight read the spell, and..." The mare bit her lip. "Well, we've been trying to figure out what happened next since we got here, wherever here is. Our memories just kind of came together in this big glob of information. Several of us think we created a sort of hive mind."
"I... see," said Celestia, who didn't.
The other mare blinked. "Really? Could you explain it to us?"
Before Celestia could answer, six equidistantly spaced lumps of magic emerged from the mare's barrel, each one a different color. They circled her as she began to rise. "Uh..."
The worried utterance broke Celestia's stupor. She was no Star Swirl, but she could see where this was going. "Focus on your individual identities! Don't let yourselves be subsumed!"
The composite nodded and shut her eyes. The magic surrounding her, now a white band of energy, shifted to a strange pastel rainbow moments before it collapsed back into her.
There was an incredible, brilliant light, too bright even for Celestia's eyes. As she looked away, she found herself wondering if it were possible for a pony to ascend twice.
The flare faded, and Celestia stood alone on the Astral Plane. She struggled for words for a few moments. Finally, in what would be a whine from a less dignified pony, she said, "I had a song ready and everything."
Ponyville had long been accustomed to screams of horror. Sure, they had gotten more frequent recently, but the town had always sat on the edge of the Everfree. Still, a dragon doing the horrified screaming was a rather novel experience.
Spike had gotten the incoherent shouts out of his system. Now, as night fell, he was pacing outside of the library, muttering in the panicked fashion Twilight hadn't meant to teach him. "What am I gonna do? What am I gonna do? Twilight's gone! All the others are gone! Who's gonna run the library? Who's gonna rescue Equestria? Who's gonna keep Discord under control? Why hasn't Celestia written me back!?"
"Why are you doing this outside the library?" Bonbon asked from in the crowd that had gathered around the spectacle.
"Bonnie!" Lyra nudged her. "It's rude to interrupt good street theater!"
"This isn't a performance!" Spike cried. "This is a national emergency!" He looked down and twisted a toe into the dirt. "And, uh, it's a lot easier to fill in a ditch than to ask an earth pony to coax the library back into shape after somepony worries a groove into the floor. Speaking from experience."
Lyra nodded. "Sensible."
Bonbon raised an eyebrow and looked at her. "Screaming his lungs out is sensible?"
"He'd be doing it either way."
"Look!" cried Blossomforth, pointing up.
All eyes followed her hoof, seeing the new constellation that shone like it was upstaging the moon. It grew larger, and the crowd could see that five of the stars were orbiting the central one.
"They're falling!" cried Daisy.
"We're all going to die!" added Roseluck.
"What is life?" said Lily. The other florists managed to tear their gazes away from their doom to stare at her. "My therapist recommended existentialist literature as a way to gain perspective on the little things."
As the stars got closer still, it became clear that only one of them even resembled a star. The others were familiar, though the townsfolk were used to seeing them as still images. The butterflies didn't normally flap, the diamonds rarely sparkled, and while the cloud did thunder sometimes, it was for much different reasons.
The symbols touched the earth with a flash of light that seemed bright as the sun. Spike's eyes were the first to clear, and he beamed as he cried, "You're back!" He charged straight towards Twilight and hugged her foreleg. "You're back!"
As far as first sensations after getting your physical form reconstituted go, thought Twilight, getting hugged by your assistant/familiar/brother/son/it's-complicated was pretty high on the list. She returned the hug. "I'm glad to be back, Spike."
Rainbow Dash cleared her throat from overhead. "Uh, guys? Hate to break up the touching moment, but we kind of have a problem on our hooves."
Twilight looked up. "What kind of horn!" Her wings flared out. After a moment, she looked at them. "Wings!"
"Hoof!" Everypony turned to Pinkie Pie, who was staring at her own leg like she'd never seen it before. She smiled. "What? It seemed like a fun game."
"Pinkie Pie is an alicorn." Twilight giggled and stumbled a little, her breaths growing shallow. "Fine. That's fine. We've already released Discord. Sanity wasn't long for this world anyway." Spike held up a paper bag. "Thank you, Spike." Twilight began to breathe into it.
"So, yeah," said Dash. She swept out a hoof, and Twilight took in a good chunk of Ponyville bowing to them. Fluttershy had hunched in on herself, Applejack wasn't looking much better, and even Rarity seemed uncomfortable with the situation. "Problem."
Twilight took one last comforting bag-breath. "Everypony, please, get up! We aren't princesses!"
"Actually..."
Everypony turned. They were sure Celestia hadn't been there a moment ago, and there definitely hadn't been any kind of flash or noise to indicate a teleport. And yet, there she was, unable to make eye contact with any of them.
Twilight felt a growing pit in her stomach. "Princess, please, please, please don't say what I think you're about to say."
"The law is clear on the matter, Twilight Sparkle."
Everypony turned again. As with her sister, Luna had arrived without any indication of doing so. "Those who bear the mantle of alicornhood must also accept its responsibilities."
Applejack stepped towards Luna "Beggin' yer pardon..." She trailed off. "Actually, Ah don't gotta beg yer pardon no more, do Ah?"
Luna nodded. "Indeed you do not, good Applejack. We are equals now."
Applejack put a hoof to her head, then glowered when she realized she had no hat to adjust. "In that case, Ah can tell it to ya straight: Ah already got responsibilities."
"As do I," said Rarity.
Fluttershy squeaked something that might have been a "Me too."
Dash sank to earth, her head low. "I'm guessing the Wonderbolts don't accept princesses."
Pinkie scowled. "Do you have any idea how long it's going to take for me to compile party dossiers for everypony in Equestria?"
"None of us wanted this," said Twilight. "None of us asked for it, or planned it, or..." She turned to Pinkie. "What was that about party dossiers?"
Pinkie waved her back the other way. "We'll talk about it later! You're losing momentum!"
"Right." Twilight faced Luna again. "None of us is ready to be a princess."
Luna nodded. "Indeed you are not."
"I understand, but..." Twilight blinked. "What?"
"I have tried to tell my sister such since she first told me of this ludicrous plan, but she eventually won me over after your deeds in the Crystal Empire. It would seem that she has been secretly grooming you for this role for some time now. She even had rather unsightly coronation dresses made." Luna shuddered.
Twilight's mouth hung open for a few seconds. She turned. Celestia was nowhere to be seen. Twilight gritted her teeth. "Oh if she thinks—"
"Not to worry, not to worry." Discord emerged out of Twilight's left ear, holding Celestia by her mane between two lion claws. "One naughty cosmic mechanic, here for pickup."
Celestia bit her lip, looking for all the world like a filly with her hoof in a cookie jar. "Twilight, I—"
Twilight held up a hoof. "No. We are not having this conversation here." She pointed the hoof at Canterlot. "We are all going to the castle, we are going to discuss this like the octarchs we apparently are, and we are going to make sure that that's the last time anypony uses the word 'octarch' for the foreseeable future."
Luna smirked at her sister. "Scant minutes on the throne and she's already more sensible than you. No wonder you wanted her to ascend."