The air hummed and flexed, and a manticore idly stared up through the bars of her cage, looking past the increasingly bright magical haze and towards the familiar glowing runes on the far ceiling, before resuming picking her claws at the packed dirt.
Filthy Rich looked over at Princess Luna and Princess Twilight with a nervous expression, but they were both concentrating on the spell, and the room began to warp and bend around them as time briefly turned itself upside down and everything tasted blue.
"First time?" asked one of the guards, and Filthy could only manage a queasy nod. The guard next to him tried to look sympathetic. Starlight looked visibly ill, too, but the guard nearest to her didn't say anything, simply gripping her spear harder.
"Twilight and her friends managed to walk here last year", said Starlight, between shallow breaths, and Filthy looked up to see large beads of perspiration forming on her forehead, below her bandaged horn. "Why do we have-"
"Because we follow procedure," said the guard, gruffly, and Starlight thought she saw her rolling her eyes beneath her helmet.
"...Yes, but why?", continued Starlight. The guard's eyes narrowed again.
"Tartarus isn't just a big cave under a mountain. The physical door you heard about - because we now share highly sensitive, classified information with civilians, apparently" - she took a moment to look disapprovingly at Twilight - "is a secure portal to a different plane, and visitors aren't usually allowed in and out of there; the Princesses made an exception last year because magic was disappearing from Equestria and they needed answers. Try a stunt like that today, the unicorns on guard would make ash of you, long before you got to the front steps and Cerberus ate you... Ma'am."
The magic was getting more and more intense, and it was becoming harder and harder to hear the guard's words over the increasingly loud humming sound that surrounded them.
Despite trying not to be sick, Starlight pressed on. "But we could-"
"The amount of magic that's going on out there, to keep all the prisoners inside and alive, is very complicated. So complicated that it takes a lot of power just to get to the part of Tartarus you need to be. Since so few ponies ever have a legitimate reason to go there, and because you'd need to get Princess Luna's permission and then prepare a very powerful spell to make the trip, it's usually easiest for Princess Luna to just do it herself," sneered the guard. "It's always best to have a professional handle it, not some civilian amateur."
Starlight winced. She looked to Filthy for support, but his angry glare was even more withering than the guard, so instead she closed her eyes and pressed into the floor with her hooves, just as they began to disintegrate.
There was a massive white flash, and the room disappeared altogether before reforming as a huge, dark, freezing cavern, lit by eerie magical light.
"...Welcome to Tartarus," said the guard, and stood to attention as the Princesses looked around.
Princess Twilight looked up at the cages lining the walls, rows of barred cells stretching high up towards the far-off cavern ceiling, each one occupied with a different magical creature. A cockatrice glared straight at her, and Twilight automatically flinched and tried to look away; but instead of turning the Princess to stone, the air in the monster's cage, the gaps between the bars, glowed with an intense, buzzing green magic as the creature squawked in pain and cowered in the corner.
"It's probably a good thing Fluttershy isn't here to see this," said Twilight.
"She would, no doubt, have wanted to free the creature," said Luna, as she strode forward, down a long path that seemed to stretch out off the edge of a cliff and into a wall of soft, magical light. "We have seen her dreams of the last time you all ventured to Tartarus. It is... good she did not see this particular wing."
"I know I couldn't find a way to make the separation spell permanent for these creatures, but... is it an idea you've thought about?", asked Twilight, grimacing as she saw yet more cells set into the high stone walls.
"It is," said Luna, blankly, as she looked around her, eyes flicking from cell to cell and rune to rune as she seemingly searched for something she hadn't yet found. "And supposing we were able to make the effects last forever, would you then wish to do it?"
"Well, that's really two questions," said Twilight, deep in thought. "Can you really separate out the different parts of a creature's magical nature, without fundamentally altering who that creature is? And then, is the purpose of a prison to reform and rehabilitate offenders, or to punish them? I mean, it's..."
"Yes," said Filthy Rich, exasperated, causing the guards to look over at him. "This would all be a very fascinating discussion, if we weren't right in the middle of TRYING TO FIND MY DAUGHTER!"
"I am searching even as we speak", said Luna, as Twilight placed a sympathetic hoof on Filthy's shoulder. "The enchantment that maintains this place is not a straightforward one. We cannot simply look from afar to see if there have been any new arrivals or departures."
She pointed at a bare cavern wall several thousand feet away, on the other side of a huge, yawning crack in the floor, and Filthy and Starlight both watched in astonishment as the wall flickered and then melted away before their eyes, transformed to a thin platform in the distance accessible only by a rope bridge.
"There," she said, waving her hoof at the new sight, "is where we were imprisoned when Lord Tirek briefly overthrew us. It was... not a pleasant experience."
"Why can't you just check to see if Diamond Tiara is here?!" cried Filthy, and Luna shot him a look, while the guards glared and gripped their spears even tighter.
"What you ask is not simple," said Luna, biting her lip as she concentrated on another piece of wall... which was now a corridor full of more cages. "In its true form, Tartarus shares a lot of its magic with changeling hives..." - Starlight gave an involuntary shudder - "...and only those who truly understand its ways are able to explore like this. There is no simple spell to give us an accounting of anypony, or any creature, currently imprisoned here. But with careful scrying, we can observe the reflections left on the magic of Tartarus by a particular being, whether a prisoner or an... unwilling visitor," she said, and Starlight shrank into herself again.
"And what about Diamond? Is she here? Are you seeing her reflections?", demanded Filthy, and one of the guards automatically took a step forward before Luna waved her back.
"I am not certain," said Luna. "This could potentially be a lengthy endeavor. We understand your desire to be reunited with your daughter. If she is here, we will find her, eventually."
Filthy kicked out at a pebble with his forehoof, and then watched in astonishment as it bounced against the wall before disappearing in a blue flash, while the wall glimmered and glowed and changed shape.
"Please do not kick the pebbles," said Luna, tartly.
"Are you alright, Starlight?", asked Twilight, as Luna and the guards continued to shine magical beams across the runes in the ceiling, occasionally pointing to some new corner or piece of floor that promptly shimmered and disappeared, to be replaced by new walls and pathways, while Filthy Rich just followed them around, gawping, clearly hoping Diamond Tiara would appear every time a new location was revealed. "You didn't need to come here."
"...I think I did," said Starlight, morosely, rubbing at her bandaged horn.
"It was an accident," said Twilight, reassuringly.
"That's... not what I meant," said Starlight, as she gazed around her. "I wanted to help rescue Trixie and the others, even without my magic. And I know how much Filthy demanded to come along until you gave in, so I wanted to be here to help you with that, too. But to actually be here, and to see it..."
Her voice tailed off, and Twilight gave her a quizzical look.
"This is where I should have been," finished Starlight, softly. "If I'd chosen to attack Princess Luna, and not you, this is where I would have ended up. In one of these cells," she said, gesturing at an empty cage, shuddering at the upturned food bowl, its contents long since rotted away to dust. "Not living in a big, warm room in your castle. Not as your student, and not as your friend."
"Friends will always find each other, Starlight," said Twilight, drawing her in for a hug. "You've repaid your debt to Equestria, many times over. And you're doing it again now, coming along to help fix a mistake that wasn't even your fault."
She looked around, and hugged Starlight tightly.
"You don't belong here," said Twilight, gently. "I'm certain of it."
"They are not here," announced Luna, suddenly, breaking the ponies out of their moment.
"You're sure?", barked Filthy, and the guard finally snapped.
"You will show some respect to your Princess of the Night-" began the guard, as her spear floated into position, before Luna stopped her again with a raised hoof.
"We understand your frustration, Filthy Rich," said Luna. "We... I... am sorry. But your daughter, and the other two missing ponies... they are not here."
"But you've hardly looked," whined Filthy. "How can you be sure? Shouldn't we go in and investigate? You could interrogate some of the prisoners and see what they've seen! Talk to Tirek, or Cozy Glow, or whoever else you have locked away in here who might give me some answers!"
"Those two," said Luna, loudly but not unkindly, "are locked away in the deepest depths of Tartarus, where no spell - no matter how accomplished the caster, Starlight Glimmer," she said, without looking at her, "- could penetrate accidentally. They would not have seen anything our prison did not wish for them to see."
"Shouldn't we at least check, though?" asked Starlight, and when Filthy looked over to her - for the first time since the accident - Starlight thought she saw something other than a hateful glare.
Luna frowned in thought, and her eyes glazed over for a moment before she returned her focus to the ponies around her.
"No," said the Princess. "No new magic has brought anypony past this point, of this we are quite certain. To venture further in would be futile, and, indeed, interfering with the magic hiding those prisoners from us would only risk the possibility of escape for some of the most dangerous villains Equestria has faced over the past two thousand years. We... I... cannot countenance such a risk when there is no chance of success." She looked at Filthy, who seemed like he was about to burst into tears.
"I am sorry, Filthy Rich," she said, with some sympathy. "But it is time to return to Ponyville, and look for new avenues of investigation."
"New avenues?!," spat Filthy, and the guard again took a step forward.
"Indeed," said Luna, ignoring the businesspony's threatening attitude. "You, and Princess Twilight, and Starlight Glimmer, shall have the whole support of our royal guard in finding your daughter, beginning with these two fine ponies here."
The guard's eyes bulged out. "Your Majesty...?" she stammered.
"We are willing to put all our resources at your disposal," continued Luna, ignoring the guard's protestations. "Fear not, Filthy Rich. We will find the missing ponies. We shall search in dreams, we shall send out search parties, and we shall find them."
Luna nodded at Twilight, and their horns began to light up as Filthy and Starlight took up their positions between the guards.
"I still think we oughta talk to Tirek, or Cozy Glow," said Filthy, under his breath, as the return spell began to take hold.
"No," said the guard. "...Sir," she reluctantly added. "Princess Luna's in charge here, and she knows more about this place than any pony in Equestria. If she says there's no need to continue, you need to trust her. And even if you don't trust her, we're not getting any further in unless she says so, so I guess you're going back to Ponyville."
Filthy looked despondent.
"...Besides," said the guard, "it's not like they're going anywhere..."