Celestia peeked over her shoulder as she went down the hallway. "Where's the last of your party?"
"Hm?"
"I remember there being three of you." She paused and turned to point at the two remaining. "Where did that interesting crystal pony go? I never heard of that tribe before..." There was no such thing as crystal bat ponies.
Morning scoffed. "Mother's conquests too intimidated her and fled for her safety."
Celestia cocked a brow at that. "Is that so? I should be careful then. Conquests make me nervous at times as well." But she turned back around. "This is the room." She willed a door open, a guard standing at either side of it. "We keep ponies out, for their safety. It could be very easy for them to become lost, or hurt."
But she was Celestia, and they did not try to stop her, or her guests, from entering. There were countless cloth bumps decorating the room. "I hid the mirrors under these." She nudged at a tarp. "Each leading to one of those worlds I spoke of. Perhaps one to yours?"
Umbra wrapped her magic around Morning, squeezing gently. "Now is the time for you to stay close."
"Yes, mother," she promised obediently. "Is there a mirror where my people are still alive?"
Umbra flinched at that, but... "Probably... But they wouldn't be yours, exactly. They had to be a little different. Before the attack? Or they never launched it? Or they won?"
"Hm." Morning inclined her head left and right, considering that. "Then there would be another Morning there. Were I to see myself, there would be violence. We would both know there can only be one of us."
Celestia shook her head. "We should not visit violence on these other worlds. Why... imagine if they returned the favor? No violence, kindly. We are guests." She pressed on through the room. "The ones we pass now are the... basic ones, the ones that seem familiar, or at least recognizable. The ones that peer yet further, we have hidden away." She willed a new door open, starting down some stairs. "It can be confusing, even jarring, to look at these. At worlds so distant, they can boggle the mind."
Umbra's ears perked. "The possibilities are endless." But she looked interested, drawn, not scared. "Thank you for inviting me here, Celestia."
"You've earned this much." They reached the bottom and Celestia turned a knob, entering a combination that allowed the last door to open into a dusty room of lumps. "The Forbidden Mirrors. Considering they're all forbidden, this is a step beyond. Now, from what little I've gathered..." She turned slowly and veered to the left. "Your people are furless, like where Sunset fled..." She sighed gently at that. "Those mirrors..." She pointed at some lumps of cloth. "I had them gathered here."
Umbra sped up towards the mirrors that promised views of human-occupied worlds. "May I?" When Celestia nodded, Umbra flipped the tarp off the first one and let it fall aside. "Let's have a look."
Inside was an office. People huddled in their cubicles, typing busily with defeated eyes. There was no joy, only misery, accepted with each stroke of their fingers. Celestia cringed and stepped back slowly. "I hope not that one... As I said, this room has more... disturbing visions."
Umbra inclined her head. "This is a bit over the top." She leaned in though, looking from one person to the next. "But right species, I think..." Two legs, two arms, fingers, right number of them. The ratios were about right. One stood up to go to the bathroom, no tail! They were humans? So far as Umbra could-- One sneezed, but the sound came from the wrong end. "Oh..." She put the tarp back over the mirror.
That didn't stop her. There were more tarps with more possibilities. "Now, you said you couldn't count them." She bobbed a hoof from one tarp to the next in the human section. "Does that count this smaller section? There doesn't seem to be many here."
Celestia's horn glowed as she yanked one tarp free, revealing two mirrors close to one another. "They can be tricky."
Umbra blinked. That lump sure looked like one mirror! "Well! Fine... Actually, that's good. That means even if it looks like there's only a few, we still have a smaller infinity. Still infinity."
Morning blinked gently. "Mother, if it's infinity, how can it be small, or large?"
"Infinities are funny like that," half laughed out Umbra. "For example... Do you know what an integer is?" That got a blank look. "A number, with no fancy extra bits. One, two, one hundred, one million and four. Those are integers. With me?" Morning nodded, watching her mother. "Alright, so, that's infinity. You can always add one, forever, and get a new integer. There is no end."
"Alright... How is that large or small? It's infinity." She spread her hooves. "It's always very big."
"Right, but now... Let's say I only count even numbers. Two, Four, Twelve, One Hundred... They never end. You can always add two and get another. Infinity!"
"Infinity," agreed Morning. "When do we get a small or large infinity?"
"We already did," smugly concluded Umbra. "Even right from the start. two, four. But in the same space, you could fit one, two, three, and four. These infinities are clearly not the same. One is bigger than the other. One contains the other, and then more stuff. We can't hope to measure either, but we know, for sure, that one is bigger than the other." She threw her hooves wildly as she talked, trying to explain that math concept to her child. "Get it?"
Celestia was watching with wide eyes. "What? When did you become a mathematician? There are... ponies struggling to put words to these concepts right now."
Umbra colored at that. "I cannot take credit for that!" she wailed. "Don't ruin their days... I'm a cheat sheet from another universe." She waved at the covered mirrors. "Not remotely fair, and not a mark of me being super clever. You understood what I said? Then you're just as clever as I am."
Celestia smiled gently. "You are modest... How did you end up taking his... Nevermind. We should not question such things." She inclined her horn at the two exposed mirrors. "Did you want a peek?"
"Yes." Umbra willed one mirror aside. "One at a time." With a clear view, she peeked inside to see what was a strange place. They were humans, but their heads were wrong. Each wore a big mascot helmet, a goofy animal face thereon, but the animal face could smile, frown, blink, and otherwise react. That was their head. "Trippy..." That was clearly not her home. She slipped the tarp over it. "Now... even if I did find mine... pretty sure I wouldn't go into it."
Celestia hummed at that. "You mentioned this before. If true, why are you so curious to look? You don't have to, I remind."
"Don't have to..." Umbra slid in front of the next mirror. "But it's there and this is, to be blunt, fascinating."
The next mirror was not a scene. She was not watching people living their lives, oh no. With a sharp movement, a hand collided with the mirror from the other side, fangs exposed in a snarl. Some human-like figure was snarling at her with menace in its blood red eyes. Was that a vampire? Ghoul? Something of the sort... "Is there a reason it didn't just spill out into this room?"
Morning had her hackles raised, looking like a little cat ready to battle. "It doesn't dare!"
Celestia willed the tarp over the mirror. "Small blessings. When one looks into a world that is actively hostile, a barrier stops anypony from going in either direction. Now... if you insisted, you can push past it, then all bets are off. I don't recommend that." She nudged the mirror back with the others. "That is clearly not a safe place."
Umbra considered from one lump to the next. Each was equally likely to hold something interesting, or horrible, or both. "To think... I fell between that space." She wobbled a hoof between the mirrors. "From one world to another. Fortunately, this seems like a nice one." She pantomimed throwing off a tarp, her magic wrenching it free of a mirror. "And what do we have here?" Snow. A lot of snow. "I don't... see anyone, furry or not..." She leaned in to look about closer. "You sure these are all the furless ones?"
"At least at the time we arranged them," assured Celestia. "But with infinite mirrors... a mistake could have been made." She watched from behind Umbra. "That is curious... I don't see.... anything but snow."
Umbra shivered. "I'm getting cold just looking at it." But, there, in the distance. Umbra squinted for a better view. There was a fire, a tiny, distant, fire. Next to it, a single person. Male? Female? She couldn't tell, not at that distance. Besides, they were wearing thick clothing, as one would have to for that kind of inclement weather. It was a single person surviving in the middle of the arctic wasteland... "Wow..." The end of a world? A brave explorer? Part of her wanted to go and ask... "What happens if I go through?" She pointed at the mirror.
"That isn't your home, is it?" Celestia inclined her head at what was just a wintery hellscape from her angle. "Why would you want to visit that one?"
Umbra pointed at the long away person. "There's a person in there... I want to ask them what happened, or if they're just... exploring, or... I don't know... Curiosity is burning. Can I go ask?"
Morning lifted an ear, gazing on Celestia with new respect. "You must wield a terrible power, to make mother ask you before doing things."
"Vast and mighty," agreed Celestia gently. "But we are friends. I don't wish to stop you... but these portals can be... spotty. It may be fine, or it may close in a minute, or an hour... One can never be certain of their timing unless a given mirror is closely studied, which this one hasn't." She ducked down to look at something near the base. "No... not studied yet. We put plaques on the ones we figure out, for future reference. So... It is a risk... you may be stuck there for some time."
Umbra clapped once. "Ah! Is that how you knew how the other one worked? You'd already studied it... But why did you study it?" She looked between Celestia and the mirror. "It was just a random mirror, wasn't it?"
"It... was not," admitted Celestia, sagging. "Sunset Shimmer was my student... prized and so talented. Like Twilight, in a way... but Twilight came out of her shell and spread her wings, literally. Sunset instead ran away... One of my great failings." She sighed slowly. "I knew which mirror she fled through, so we studied it, and we knew it. That is also why we gathered all the furless mirrors together."
Umbra hummed gently. "That person..." She really wanted to know. "If it's as desolate as it looks, they may be the last... It wouldn't be... against any rules to offer them a different ending? If they are the last, the story of their people in their world is already over, just waiting for the last page to fold shut." She brought her hooves together with that finality. "It would be a mercy to offer that very last one a chance for some happiness, and wouldn't change the story of that world."
Celestia frowned with thought. "We do not want to unbalance the worlds... But if they are truly the last, a doomed last breath... then we wouldn't change much..." She took a slow breath. "Is it fair? There are countless worlds by your own logic, just like this one. Why rescue this one?"
Umbra smiled gently. "Because this is the one I saw."