"Don't you think you should wait for Starlight to come back from her trip?" Spike asked nervously.

Twilight laughed. "Don't be silly, Spike. I've been doing magical research for years with just your help. I don't need another unicorn to spot me."

"Yeah, but..." His eyes flickered over the stack of books, the materials, the preparations. "You yourself said this is a completely new spell. Nopony's ever done this before."

"That might not be true. The whole reason I got the idea was those books from the Crystal Empire I've been reading." She lifted one in her magic, taking a few merrily prancing steps toward it to meet it halfway. "Look, here, this is the first hint I've found in any record as to where Starswirl went, and it strongly implies he opened a dimensional portal. 'Then to defeat the evil did Starswirl banish himself and the Pillars to a world of darkness.' How do you banish yourself to another world without opening a portal?" She put the book down. "Discord insists that he's the one who created the gateway to the world Sunset's in, and all Starswirl did was anchor it into the mirror and put in containment spells to keep it from opening up just any old time, and after I saw what Discord did at the Gala that one year, I believe him. But Starswirl himself must have figured out how to create portal spells himself, because Discord says he doesn't know anything about this banishment to darkness thing and he himself was in another dimension when it happened, so he couldn't have created the portal in this case."

"But you haven't found any evidence of an incomplete spell in the Starswirl Wing or in the Crystal Empire."

"Right, and that's why this is so exciting!" She reared back on her hind legs and kicked her forehooves a little, unable to contain herself. "This will be new magic! Completely new magic, not like fixing a spell somepony else started!"

"And that's why I think you should wait for Starlight! She has experience creating completely new spells."

Despite her best efforts, Twilight's mood soured slightly. "Yes, her cutie mark removal spell. That was groundbreaking. Plus, she fixed one of Starswirl's spells herself when she got the time spell working. Starlight's amazing. That's why I want to do this now."

"You want to do this without her because she's amazing."

"Yes!" Twilight paced. "Spike, I had all the advantages. I was Princess Celestia's student. I had the run of the library. My job was just to learn magic, for ten years. I was so good at magic I became an alicorn. And Starlight, who never went to Princess Celestia's school at all, who didn't have nearly the same access to books of spells and theory that I did, managed to both fix one of Starswirl's spells – which is the thing I did that made me an alicorn – and create completely new magic from scratch, which I've never done. Don't you see, Spike?" She stopped pacing and looked at him. "She's better than me! At the thing I'm supposed to be an expert on!"

"I thought you were supposed to be an expert on friendship," Spike said. "And anyway, Starlight is ten years older than you. She just had a lot more time to learn magic and practice it."

"Yes, I'm supposed to be the friendship expert, but who was it who reformed the entire Changeling race? Starlight!"

"Actually I kinda think it was Thorax..."

"Well, okay, but because of advice Starlight gave him!" Twilight sighed. "I'm not jealous of her, don't get me wrong. It's not good to be jealous of a friend."

"That didn't stop you during the whole thing with Discord and our friends after the book sortation..."

Twilight ignored that. "She solved a friendship problem between the Princesses. She's... maybe not perfect, but she's mastered this whole friendship thing and reforming ponies after just one year of me teaching her. It took me three years to get to the point of becoming the Princess of Friendship! Four, before I figured out that was what I was princess of!"

"Starlight's still not a princess, and have you considered that maybe she learned faster because she had a teacher, and you had to do all the research on your own with your friends' help? I mean, why are you looking at this as 'Starlight learned about friendship faster than I did' when you could look at it as 'I taught Starlight about friendship so well she could reform the Changelings in only one year of study, when she started out as a supervillain?'"

"I'm not sure Starlight ever qualified as a supervillain, that's kind of a comic book thing—"

"She destroyed the world seven times, I think it counts."

"Okay, okay, I know what you're trying to say and I understand. And I shouldn't be jealous of Starlight, and yes, she's older than me so she's had more time to learn magic, but..." Twilight sighed. "Magical research used to be what I was known for. It was what I wanted to do with my life. This whole princess thing? I never expected this. I wouldn't change it, but... I don't feel like I'm really me anymore. I never get to do what I love, the first thing I was good at."

"I thought the first thing you were good at was reading."

"You know what I mean." Twilight brightened up. "So that's why I'm going to open a portal to another dimension, without Starlight's help, and prove beyond a doubt that that's a thing harmonic magic can do!"

"Okay, but if you won't get Starlight's help... don't you think at least some unicorn should be here to spot you in case something goes wrong?"

"I don't think Rarity or Sweetie Belle would be much help. Or anypony else in Ponyville, really. And besides, I have a lot of experience with this!" She raised her forehoof and pawed at the air, making imaginary check marks on equally imaginary ticky boxes. "Firstly, I've opened dimensional portals before. I was in the Realm of Chaos at the time and it's a lot easier to do it there than it is to do it here, but I know how it feels. Secondly, I was right there at the Gala when I saw Discord open a portal, and... the thing about chaos magic is that it's raw magic. Discord's so much more powerful than any of us because he can just talk to magic and get it to do what he wants without using any of the energy in building a constraint structure. But once he gets it to do a thing for him, I can see how the thing is built and figure out the constraint structure that would be needed to make it happen. Plus, thirdly, my counterpart in Sunset's universe opened a lot of dimensional portals without any trouble when she'd absorbed all the magic from my friends over there... which means she was probably about alicorn level at the time."

"I thought there was a lot less magic over there overall? Can anypony – sorry, anyone over there get to alicorn level?"

"Well, if she wasn't alicorn level then it's even more obvious that I should have enough raw power to do this!" Twilight gestured at the place where she was going to bring a portal into existence.

Spike sighed. "All right, fine..."

Twilight took a deep breath. It had long been held by the greatest minds in Equestria that it wasn't possible for harmonic magic, the type that unicorns and alicorns cast, to open dimensional portals, and that Starswirl had taken advantage of naturally occurring ones. Except Discord had confirmed that that wasn't true. A naturally occurring portal would have still been around for Discord to find if Starswirl had used it to banish himself, or the evil, or whatever it was that that passage was actually claiming he did. In fact, it had turned out the main reason Princess Celestia had wanted Discord reformed was his power to find and close the portals. Starswirl's had to be closed or Discord would have found it by now, and if Starswirl could close a portal, odds were he had opened it in the first place.

So Twilight's theory was that yes, harmonic magic could open portals, and that she could devise a spell that would do so.

She was about to make history. Well, okay, she'd made history several times by now, but she was about to do it again. Twilight checked her saddlebags once again, making sure that everything she'd planned to pack was still in them. Once the portal was opened, after all, she definitely wanted to go through it and explore the world on the other side, as long as it was safe for equine life over there.

She concentrated, shaped the spell in her mind, and fired it from her horn.

A distortion wavered in the air in front of her. Success! It wasn't something she could clearly see through, like the portal Discord had opened at the Gala or the one bound in the mirror, but it was definitely there. "YES!" she shouted, excited. She started to trot over to the distortion to stick her head in and see what it looked like on the other side, but common sense prevailed – once the horn that created the portal went through it, it might just close and spit her entire body out on the other side. "Spike, can you stick your head through and see what it looks like on the other side?"

"Sure." Spike walked toward the portal. "Uh, Twilight?"

"Is something wrong?"

"It feels like... something's pulling..." Spike tried to step back – and fell on his tail instead, whereupon he slid forward toward the portal. "It's pulling me in!"

"No!" Twilight grabbed onto him with her magic. The portal was definitely yanking at him. And at her magic. Her grip on Spike was weakening. It felt as if her magic was draining away from where it contacted him, toward the portal.

She charged forward and grabbed Spike's tail with her hooves, trying to use the earth pony strength of an alicorn to pull him back, but she had no traction – the crystal floor was smooth and polished, nothing to stop either of them from sliding forward. She tried using her wings, but it was too late. With a pair of screams, they both slid the last little bit across the floor and through the hole into another world.

From his home in Chaosville, Discord couldn't necessarily detect all the ebbs and flows of magic in Equestria, but something as significant as a dimensional portal made him sit up and take notice.

He'd been playing a card game with himself in which he changed the rules constantly. The other Discord adapted to the rule changes instantly, of course. It was a semi-detached alternate, like Specs but with less personality, so that he wouldn't immediately know its cards. But it wasn't very smart or good at this game, so it wasn't much of a challenge... or, honestly, much fun. This promised to be much more interesting.

The other Discord and the cards popped out of existence, and Discord himself appeared... right outside Twilight's castle. Oh, now, he should have guessed. If a random dimensional portal opened in Equestria, it would probably be either Twilight Sparkle or Starlight Glimmer's fault, and they both lived here. He couldn't teleport into the castle because of the harmonics, but he could certainly locate the portal. Quickly he flew there, already thinking of the funny things he'd say to Twilight or Starlight, whichever one of them was muscling in on his territory.

Except there was nopony there. Just an unstable portal, floating in midair. An unstable portal that was exerting enormous negative magical pressure on the room, attempting to suck in anything that contained enough magic to be pulled by the pressure. Discord was used to this kind of behavior, and had already magically anchored himself far enough away that the pull scarcely bothered him.

He saw skid marks from hooves and dragon claws on the floor, but no magical pony and no Spike.

A quick summoning brought him a scrying crystal that had been hardened against the harmonics. It found Starlight, in the Crystal Empire, spending quality time with her coltfriend. It did not find Twilight. It would have found her anywhere in the world, which meant she wasn't in the world.

Several thoughts and emotions ran through Discord all at once. Fear for Twilight and Spike, who were his friends. Deep irritation, bordering on anger, that Twilight had been such an idiot as to try to use harmonic magic to create a portal – that wasn't what harmonic magic was good at! You wanted a portal, use chaos! Excitement at the thought of being able to rescue Twilight, rub her nose in the fact that he'd saved her, and get praise for being a hero. Unease at the fact that he could see the branes in the portal separating, growing further apart slowly. Already, the return portal would have separated from the entrance portal, so Twilight wouldn't be able to just turn around and come back. She'd have to be told to find the return portal, that it would be separated... but if he waited too long, it could splinter again, and then the closest exit portal wouldn't be a return portal anymore and Twilight and Spike could end up having to play scavenger hunt to find their way back to Equestria at all.

He could tell from the characteristics of the portal that it opened to a place with gravity similar to Equestria's, with breathable air, where the laws of physics were roughly similar to Equestria's, but where there was very, very low magic. He could even tell that the world the portal opened onto was one he'd either been to before, or its close cognate – one of the myriad worlds of humans that populated the multiverse, not the pony-like humans from the world Twilight had visited but real, fully alien humans. There was no translation matrix, so they'd be over there as a pony and a dragon... in a world where there was one dominant tool-using sapient species, and they thought of all the other species as non-sapients.

Having been mistaken for a non-sapient being in his lifetime and having suffered greatly as a result, Discord didn't handle that thought well.

A low-magic world would drain him badly and he might not have the strength to get them back directly, without using Twilight's broken portal, but he should be able to replenish quickly enough from the ambient chaos in the environment that he'd at least be able to teleport to the exit portal. He'd have to end up in a place where there was no chaos he could reach at all – deep inside a cave system or in a frozen polar wasteland or something – for it to drain him so quickly that he wouldn't be able to help them. And the thought of leaving them there – of facing Fluttershy and telling her that Twilight was gone, that all they could do was wait for her to find her way back, of leaving brilliant Twilight and snarky little Spike in a world where the denizens would think they were animals, and where animals were routinely hunted for food... no.

All of this passed through his mind in a second. Without hesitating any further, he threw himself at the portal, planning to retrieve Twilight and Spike and bring them back home.