An apprehensive Celestia took in the sight of herself reflected in a thousand mirrors. Next to her, an overly excited stallion chattered away.
"So, Cellybelly, this is the culmination of my life's work! Hundreds of portals to alternate universes, right here at our disposal. Think of all we could learn from every mirror universe out there - the technologies, the magic, the fancy hats!" Starswirl grinned, leaning too close to her in the way party ponies and eccentric old mages sometimes did. "So let's go. It's time for an adventure, tia Tia. You can take a break from your throne for a little while to go explore."
"Has this been tested?" Celestia asked, not unreasonably.
"Tiatiatiatiatiatia, I'm not in a school, I don't need to do tests. Besides, my cutie mark is in thought. I'm a thinker, Swelly. I think. When has anything I've thought ever gone wrong?" Starswirl asked, much more unreasonably.
"Well, there was the army of evil gingerbread ponies -"
"And they were delicious, after you toasted them!"
"- and the cube with twelve vertices -"
"That was a feature, not a bug."
"- and your unfinished spell."
"It makes its caster turn green now, sure, but it's called unfinished for a reason." Starswirl sunk a hoof into the nearest mirror, which rippled in a way that made some primal instinct, deep inside Celestia's mind, scream. "Look, Princeleste, you can never perfectly predict the future, okay? I mean, you can if you have a time travel spell, but ignore that. You can never predict the future, and you need to know the future to make good tests, so there's no point in testing anything."
"Starswirl, that's immensely stupid. We're leaving, now", Celestia said. She grabbed the stallion in her telekinetic grip and lifted. "I'll write up a checklist and we can come back to test these after, but I'm not letting you go through these right now. Friends don't let friends hurt themselves."
"Friends don't let friends be boring either, Boooolestia, and you're being really boring right now", Starswirl replied. He did something she couldn't understand with his horn, and suddenly their positions were reversed. She flailed, upside down, in his telekinetic grip, but couldn't break free.
"Starswirl, let go!" Celestia regretted saying that almost immediately, when he tossed her into a mirror.
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At one point, five hundred years ago, Celestia had gotten frustrated with a noble named Bluebad. For years, the mare had tried to kill her or make her miserable, doing everything from slipping exploding cigars into the Princess' personal items to trying to cut her famous rainbow mane away.
One day, when Bluebad was at a laundromat, an alicorn-guided meteor happened to fall on her head. Bluebad's house swore vengeance, and their conspiracy lasted almost three hundred years. With their members in several influential positions, they managed to secretly destroy a huge amount of records and paperwork. The amount of damage they did was estimated to have been in the thousands of bits - hundreds of thousands, if you accounted for inflation.
If Celestia had reassigned Bluebad to the Frozen North instead of killing her, Equestria would have been much stronger now.
Celestia learned from that incident. She learned that there was always a way forward, in hindsight, that did not involve hurting people. She learned that these ways tended to be harder but safer. She still fought monsters, but rarely hurt other ponies any more.
This was one of the reasons Starswirl was not on fire now.
One of the other reasons was how weak she was. She felt like she was completely drained of magic.
Starswirl groaned from beside her. She ignored him.
Around her, limestone glinted. The floor was limestone, the pillars were limestone, the ceiling was limestone. The limestone was arranged into something like Pegasopolian temples' architecture, which was worrying. It was a small step, in her experience, between ornate Peganistic pillars and the natives screaming "THIS IS SMARTA" at hopelessly confused visitors.
Starswirl groaned again, more loudly, which was good. They couldn't have been in a vaccuum or a poisonous atmosphere if the stallion was still breathing.
Starswirl seemed to realize he had air as well, because he decided to use it very loudly. "I can't feel my magic! Princess, what's going on?"
Strange and irritating as the stallion was, he was one of her little ponies, and a sort of friend to her. Princess instinct took over, and she made sure she sounded calm before talking. "You've never had your magic drained before, have you, Starswirl? I have, once. I'd been to the land of centaurs a few years ago to rescue a kidnapped unicorn. There, a Sendakin sage had drained me of my powers and left me in a state like this. Until our magic returns, we won't be able to commune with the earth or the sky or the stars."
"That's possible?" Starswirl asked, aghast. "You can actually lose your magic?"
"Yes, you can. As we've just learned, some universes even drain our magic automatically." Celestia's ears turned as she spoke. "Do you hear something? Like someone is breathing heavily, somewhere around here." She turned toward the source of the sound. "Hello? We know you're there. We mean you no harm. We're just visiting."
A thing emerged from behind a pillar, two-legged, lanky, and tall. Brown-skinned, wearing what seemed to be clothes, with a mane and beard but no coat, it looked like half a centaur painted by a five-year old Randolf Arter.
She decided to call it Cen until she learned what its name was.
Cen said something incomprehensible, a long group of sounds that no pony would ever be able to pronounce without access to translation magic. One of its forelegs reached for its middle, where a knife hung.
Celestia tackled Starswirl, knocking him into the mirror behind them, and followed him. She knew a threat when she saw one, and she didn't intend to stay long enough to find out how powerful that knife was.
On the other side of the mirror, she felt power surge through her again, and quickly cast a shield spell over the mirror they had just left. Cen wouldn't be able to follow them here.
"I'm... sorry, Princess", Starswirl said, once he had gotten back on all four hooves.
"Starswirl, what are you apologizing for?" He never apologized, not even after the gingerpony incident.
"We could have died! That thing had a weapon, and neither of us had magic, and we could have been dead in a few seconds. We've never been in a scenario that bad in any of our other adventures." He took a deep breath. "Every other time I've messed up, I've had a spell to fix it, but I shouldn't have expected to be able to do that for every problem."
"So you're saying you've learned a lesson today?"
"Yeah." Starswirl started walking toward the entrance of the cavern. "I think I'm going to head back to Canterlot now. I'll come back for these mirrors another time."
This might have been a good day. Nopony had gotten hurt and Starswirl had learned a lesson that would stick with him for a long time. Perhaps next time he'd think his inventions through. He could do so much good with his brilliance, if he made sure to test for safety first.
One of Cen's claws reached through the portal, hitting the forcefield. Celestia cast a beam of raw concussive force at it, and it was blown backward.
Yes, such a good day.
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This was an awful day.
Prince Bluebane had gone over taxes and tax breaks and financial lending for the entire day. The entire day. She had had to hear his proposals from breakfast up til dinner.
The Prince was immensely good at his job, and he was a good person, but he spoke in the most boring monotone possible. She just couldn't pay attention. The Prince would notice her attention wandering as he talked, and would start all over again. He had apparently not gotten the memo that he had his title so he could do work without bothering her; he asked her to doublecheck and double-doublecheck everything.
When she'd finally gotten to sleep, somepony knocked on her door. That the guards hadn't stopped them meant it was either an emergency or somepony she was friends with. Either way, she'd have to go answer.
She levitated the doors open, at which point a far too energetic stallion rushed in. "Princelestiaiaiaiaiaia!" Starswirl hugged her. "I did it! I figured out how to solve the mirror problem!"
"Guh?", asked the sleepy alicorn.
"So what I've done is made it so that the mirrors don't lead to real other worlds, they lead to worlds generated when someone enters. The generated worlds are the size of Canterlot, with their weather and sun and moon automatically controlled by the mirror's settings."
"Guuuh", Celestia said.
"Anyone who goes in creates a world that's like a halfway point between our world and a real other world. The inhabitants are based on ponies from our world, but take on the form and language of whatever the dominant specieses - or maybe specii, is that the plural? No, wait, doesn't matter - the dominant thingies of the real world that it's a bridge to. Anyone who uses the mirror takes those forms, too. You can take Equestrian magic in with the right settings, so you can always protect yourself, and you can copy the other world's technologies, but you can't bring their technology or magic out, so nothing evil can return. It's brilliant, if I say so myself, and I do. At least as brilliant as a giant firefly. Only problem I have now is testing them. I don't want to throw good ponies into the mirror, because they might get hurt like we nearly did, if maybe the settings don't work properly."
"I, uh, see", Celestia said, in a way that suggested she didn't see at all.
A golden stallion ran into her room, because apparently the universe hated the idea of Celestia sleeping. "Princess! There's a trio of supervillains in downtown Canterlot! They're monologuing and - and - and -"
The word supervillain pulled her completely out of her drowsiness. "And what?"
"They're singing!"
"What?"
"They're singing, Princess, it's terrible. Please, you need to stop their evil before it's too late!"
"Supervillains, you say?" Starswirl had a strangely pleased look in his eyes. "Supervillains who are being evil and need to be banished to places that aren't here? I'll stop them, my good fellow."
"Uh, my name's not Fellow, it's -"
"I don't care, just lead me to the villains I get to banish."
The two stallions left. Celestia poked her head outside her room, looking at the stallion on guard next to her doors. "Don't let anyone in unless Canterlot is on fire, understand?"
"Yes, ma'am!"
With that, Celestia curled up on her bed and started to doze off. Starswirl would probably handle the crisis downtown with ease.
It wasn't like anypony, no matter how villainous, singing in Canterlot could be that important.