It turned out that the thestrals had been imprisoned by King Sombra after Luna's mysterious disappearance. They had thought that he had had something to do with it, and so rebelled and caused havoc after the war while the Empire was trying to recover. The last thing the Empire had needed was a kick while it was already down, so King Sombra had offered them the pocket dimension as a win-win: a home for the thestrals, and extra security for King Sombra.
The thestrals had had no choice, as at the time, their kind was dying, but they resented King Sombra because they thought that he was responsible for the disappearance of their mistress. While they weren't wrong, the entire truth made them regard the king in a new light.
Sunset pondered all this and more as they made their way through the pocket dimension. It was very beautiful, currently bathed in darkness with flowing streams, lush forests, and a vibrant, bustling village of thestrals in the middle of it all. Their culture vaugely reminded Sunset of Zebrica.
"Here is the door to the other side," the head thestral - Evening Light, his name was - boomed, startling Sunset out of her thoughts.
It was plain and wooden, but its presence on the magical plane was akin to Sombra himself, which was a given considering he had made it.
"Thank you, sir," Sour Sweet said, smiling. "We're very grateful for your assistance."
"You are welcome, little pony," Evening Star said. "Perhaps when Luna and Celestia are restored we will be reintroduced to pony society. It is our greatest wish. While crafted by a master, this dimension in artificial, and it feels as much." He looked to me expectantly.
"I shall inform the rulers of your great assistance, and do everything in my power to return you to the real world," Sunset promised. She knew what he meant; it was akin to magical projections. Almost real, but not quite.
"Thank you." He swept a bat wing towards the door, and Sunset opened it with her magic, reveling the rush it gave her to intertwine such ancient power with her own. The door swept open, and then they were on the other side.
"Wow," Lemon breathed, and she and Sunset exchanged knowing looks, continuing on their way.
"Sunset?" Sour Sweet asked, coming up beside her. It gave Sunset mild surprise; Sour Sweet seemed the quietest of the group. "What are the Elements, exactly? What do they look like? What do they do?"
"Well, there are six elements: Kindness, Laughter, Generosity, Loyalty, and Honesty. The sixth is unknown, but it is what binds them together and links the power to be used. It is also - supposedly - the most powerful. I don't know what they do, exactly, but they're supposed to be big spheres made of marble with and engraving made to look like gemstone on them. Remember, this is all specluation, hypothesis, and inference, so I may be totally off the mark."
Sour Sweet nodded. "Okay."
In one hallway, the mares came across a foal.
A foal!
"Stay back, guys," Sunset warned. "This could be a trap."
"Oh, pish-posh," Sour Sweet said. "It's obviously a foal, and it needs help!"
"Sour's always been a big softie," Indigo whispered to her.
Sour cleared her throat. "Excuse me? What are you doing all the way down here? You are in grave danger."
The foal sniffled, looked up, and squeaked. Before our eyes, it transformed into a potted plant, with a flash of green flame.
Sunset gasped, along with everyone else. Sour looked on the verge of fainting, but held her ground. "You are in grave danger," she repeated, and held out a hoof.
The plant's flower looked up, and transformed into what Sunset assumed to be its natural form. Her stomach almost revolted in disgust.
It appeared to be and alicorn of normal size, but . . . buggified. It was all black with holes in what the functional part of Sunset's brain assumed to be chitin, with huge turquoise eyes. Its wings were clear and also had holes in them, appearing to be a tattered joke of a bee's wings.
Sour Sweet still held her ground.
"Are you mad?!" Sunset hissed to her. "It's disgusting!"
"Kindness is universal," she said firmly. "It needs help, so we help it. If it betrays us, we are more than equipped to destroy it."
Sunset couldn't help but admire Sour Sweet's strict moral compass. She was sure even Sombra wouldn't be this accomadating - which was exactly why it was a stupid idea.
"Fine."
The thing took Sour Sweet's limb with a trembling hoof and got up. "Thank you," it whispered. "I am Thorax. Thank you so, so much."
"What are you doing down here, Thorax?" Sour Sweet asked, putting a kind hoof over his back.
Thorax gulped. "My division got wiped out," he said. "With the return of Nightmare Moon and Daybreaker, we figured we could align ourselves with them to take over Equestria - don't look at me like that, we were taught from grade school that ponies were monsters and not to be trusted. There's a small part of me that's screaming at me to kill you and run, but with all the emotions rolling off you, you're probably genuine."
"Emotions?" I butted in.
"We feed on emotions. Love is the most filling, but we can eat pretty much anything. We can also sense them. Anyway, my division was searching this hall for the queens when something shot out of the walls, killed all of my division, and took the bodies with it." He shivered. "I am the only survivor."
The group's eyes were wide with horror. "I can't imagine what you've been through," Sour Sweet said hoarsely.
Thorax gulped. "I'd been lying there for the past hour, trying to convince myself to get up, but each division has a strong mental link, always buzzing with chatter. It's a way to scare our enemies - movements perfectly coordinated but no sound from any of us. That link was still established when they got killed. You don't know . . . you don't know how it felt to hear that link go silent. The last words Pantala said to me were that she loved me, even though it was forbidden. I kept on wondering where that extra food had come from, and I loved her too."
A tear rolled down his cheek.
"And Pharnyx . . . I never got to tell him that he was the was the best brother anyone could ask for."
Silence, and suddenly everyone started hugging him, Sunset included. No monstrosities like that had ever been committed during Sombra's rule, and none of them had ever heard such a horrifying story before. Thorax blinked. "You ponies literally vomit love all over the place."
"We're sorry, Thorax," Sour Sweet said sincerely.
"Thanks. Anyway, what are you doing down here?"
Sunset filled him in. "We want to go to the deepest vault here, but I'm not sure we're going the right way. The pocket dimension kind of turned me around."
"We memorized a blueprint of the castle before we came. I could take you."
Sunset smiled gratefully at him, and took a deep, shuddering breath. She had been on the verge of crying herself. "Lead the way."
"Why were you nice to him, even after the rest of us told you not to be?" Sunset asked Sour Sweet.
Sour Sweet shrugged. "It was just a feeling I had, in my heart of hearts, and that's where I look when making a big decision: not my mind, not my instinct, my heart. I find that's where the best decision is often located. Everyone deserves a little kindness. Besides, what goes around comes around."
Sunset smiled.
They stopped at another door, and Sunset started to use her magic to pull it open when she realized that it was stuck. She pulled harder, and it still wouldn't open.
Then, a mist seeped out from under it, forming a pony figure.
"This was not in the blueprint," Thorax muttered to himself.
Sunset wasn't scared; she was just exasperated. The night had given her too much crap for her to really care anymore - or was it day? The sky seemed to be stuck in the eclipse, so she really didn't know.
So, as was her reaction to many things the night had thrown at her, she cursed.
The spirit chuckled. "Such spirit! I like you. Unfortunately, I am not here to make small talk. To pass this doorway, I must claim a sacrifice. A life would be good, or maybe eternal servitude, to perhaps just a little old-fashioned torture-"
Here her smile went slightly off-kilter.
Now Sunset was really scared.
Sunny Flare stepped up. "I'll do it."
"Well, I've gone with the life choice. Do you still want to be my sacrifice?"
"NO!" everyone cried, and Sour Sweet downright burst into tears. Sunset had grown to respect everyone in the group, too.
"Yes," said Sunny, casting an annoyed glance at all of them. "Though, I'm not too keen on the dying part. Perhaps we could make a trade?"
The spirit looked intrigued. "What do you have to offer me?"
Sunny lifted her hoof - there was a priceless bracelet on there, the beads all perfectly round amethysts that matched her mane. It was beautiful. "This is a family heirloom, passed down since the Great War. It's very dear to me. Is it a worthy trade?"
The spirit smiled. Again, slightly off-kilter, but this time no one cared. "I'll take it!"
"Are you sure, Sunny?" Indigo asked urgently.
"Anything. Besides, it would look good on her!" She smiled encouragingly. "After all this is over, I could procure more accessories for you!"
The ghost smiled, a real one this time. "Thank you," she whispered, reverently slipping the bracelet on her hoof. Then she disappeared.
"Are you crazy, Sunny?" Indigo asked.
"Of course not! Plus, look at it this way: that ghost probably hasn't had a single nice visitor to her in a thousand years. Anyone would go a little insane after that. I don't really want to see her again, but who knows? Maybe I could be her friend, bring a psychologist down to look at her. It would do wonders."
"That's really generous of you," Sunset said.
Sunny shrugged. "Anyone would do it."