"So, in your world, you all have these 'cutie mark' things? For your special talent?" Queen Celestia and Twilight Sparkle were walking through the halls of Canterlot palace, with Spike riding on Twilight's shoulder.
"Yeah. They're on your flank, and you usually get them when you're around 10 or 12 years old. What do you have instead?" Twilight asked.
"The closest parallel I can think of is our sigils. But... you say everyone gets the marks? Automatically?"
"Well, how else would it happen?"
Celestia sighed. "You really don't have the Worldspinner, it seems."
"Who is the Worldspinner? Is it some sort of creation story?" Twilight asked.
"Of a sort. She is more of a presence, guiding us and our fate, giving us our magic and our purposes. If you truly wish to know more about her, I could take you to one of the temples." Celestia stopped in front of a large mahogany door. "Here is the Canterlot archives. It will tell you all you wish to know of our world." She used her magic to open the doors, and within a thousand books and scrolls seemed to sparkle with unfound knowledge. Celestia started to speak again, but it seemed muffled in Twilight's ears. All that mattered was the knowledge that existed in that room. A whole parallel universe! Who knew what was different? She would have to take notes, make charts, diagrams...
"...Twilight? Hello? Earth to Twilight Sparkle!" Spike was waving one of his claws in front of her eyes.
"Oh! I'm sorry, Spike, it's just... this is so amazing! I can't wait to get started!" Twilight sounded like a little filly again, bouncing on the tips of her hooves.
"We're on a time limit here, Twilight. Remember? The portal is only open for three days!" The little dragon climbed down, sinking onto his haunches. "We need to find Sunset Shimmer and get the crown back!"
"Sunset... Shimmer?" Celestia tilted her head. "She's back?"
"Back? But she was our Celestia's former protege!" Spike exclaimed. He narrowed his eyes. "Have you been teaching her while she was in your dimension?"
"Your Sunset jumped dimensions too?" The queen seemed genuinely surprised. "Our worlds parallel each other in more ways than one, it appears." Her voice suddenly took a darker turn. "If your Sunset has the magical prowess of mine, we need to find her and stop her immediately. She had raw power like few I had ever seen. The only unicorn who could match her was..." she turned to Twilight. "...you."
Twilight was taken aback. "Me? You taught the alternate version of me?" For the first time she thought of her friends and what they were like in this place. "Is she an alicorn too? No... she can't be. When you interrogated me you didn't expect me to be."
Celestia sighed. "No. I feared her becoming one, so I sent her far away from me. It was hard, but I knew that it was for the good of Equestria."
"You feared me becoming an alicorn? But... you - my version of you - groomed me for this. Did Cadence and Luna think I shouldn't have been one either?"
A jolt seemed to go through Celestia's body. "...How do you know about Luna?"
"What do you mean? Isn't she around here somewhere?" A suspicion started to grow inside of Twilight, but she pushed it down, refusing to believe it.
"I don't know what happened in your world, but that is not what happened here." Celestia's voice was steely and hard.
"...how did you defeat her?" Twilight whispered, fearing the answer.
"She is imprisoned deep under Canterlot, in a prison magically fortified with the best defences our mages could conjure. Please, do not speak any more of it." Celestia turned away, letting a lock of flowing hair cover her face. "I will get someone who will assist you in your search for Sunset Shimmer. In the meanwhile, please search the archives to understand this world." The queen turned and walked off.
Twilight sighed, and turned to enter the library, sadness in her eyes. Spike put a claw on her side in silent comfort.
~~~
Books, scrolls, and papers covered the large desk that Twilight was sitting at. Her levitation magic was working overtime as she scribbled notes on multiple pieces of parchment. She had several books on front of her, peering from one to the other. Her mane was frazzled, sticking up in different directions. Spike was slinking through the bookcases, until he found a particular tome. He grabbed it and waddled over to Twilight. "How about this one? It's supposedly an ancient study on the... Well, I don't know what it says, but it says something about Starswirl. D'ya think it'll help?"
"Add it to the pile, Spike," Twilight barely even looked up. "I'll get to it eventually."
"Celestia's been gone a long time now. When do you think she'll be back?" Spike sat on the table, and curled his tail around his claws.
"...Do we want her to come back?" Twilight said tentatively.
"Why wouldn't we want that? We need her if we're gonna find Sunset," Spike pointed out.
"Spike, she's hiding something. I think she's... afraid of me. What did the other me do to make her act like this?" Twilight sighed. "I've been reading on differences in our histories. Equestria is really suffering without my friends and me." She turned to her notes, levitating some pieces of paper. "Nightmare Moon was brought down by Celestia harnessing her shadeself with the assistance of a group of mages who reharnessed her after Nightmare Moon was imprisoned. Isn't that brutal? Look how sad it made Celestia."
"Twilight, what's a shadeself?" Spike asked.
"Oh, right," she levitated another sheet. "Basically, every powerful magic user has a sort of... corrupted version of themselves. It's more powerful, but also has skewed morals and the host pony has no control. The more powerful the magic user, the more unhinged their shadeself is. Nightmare Moon is Luna's shadeself, and she's been trapped in her for over a thousand years now. According to this account, Celestia's shadeself is some terrifying mare named Daybreaker, who was extremely hard to bring down after Nightmare Moon was restrained. It seems likely that the longer one exists as their shadeself, the harder it is to change back."
"Hm. Interesting," Spike remarked. "Hey, I wonder what your shadeself looks like, Twilight."
She shuddered. "Let's hope we never find out." She pulled out a different page of notes. "Moving on, next up is Discord. Apparently he was banished to a different dimension by an artifact of by Starswirl the Bearded. I hope we don't have to travel to that dimension and fix all the chaos he's caused there. Chrysalis' defeat wasn't too different apparently, except for the fact that Cadence became an alicorn during it. When the Crystal Empire came back, get this, it says 'Twilight Sparkle, Princess Cadence's protege, and her dragon, assisted the new alicorn in defeating King Sombra.' I'm apparently Cadence's student in this world! I wonder why I wasn't sent to Ponyville."
"So you're saying Spike the Brave and Glorious still exists in this world?" Spike smoothed back his spines and peered at his claws vainly.
Twilight giggled. "Yes, I'm sure you're still a hero to the Crystal Empire." She was about to go back to her notes, when there was a knock at the door.
"Yes? Come in!" Twilight called out.
The door creaked open, and Celestia's tall visage nearly blocked out the short blue pony near her hooves. When Twilight saw who it was, she gasped.
"Rainbow Dash?"