Author's Note
And it's done! 
This is my first posted fanfic ever and English isn't my native language so please have that in mind while reading this story. I'm open to feedback and constructive criticism 
Feel free to let me know, if there's anything wrong with my story. Also, as I have never posted any fanfiction before, I'm not entirely sure if I tagged this story correctly 
Rainbow is not convinced
“That. Was. AWSOME!” Rainbow Dash exclaimed, throwing her arms into the air when she and her six friends came out of the cinema.
“It sure was-” Pinkie sang.
Rarity looked at her. ”Um... Pinkie Pie, dear, why do you have popcorn... in your hair?” she said slowly, but was ignored.
“Agreed. T'was sure somethin' they've got 'ere,” Applejack motioned the building they were leaving behind. “Nothin' like some good ol' MARVEL movie with loads of kickin' thems' bad guys' butts!”
“A little too much for my liking,” Fluttershy muttered. As usual, nobody heard her.
“But the plot has holes huge like the Mariana Trench,” Sci-Twi pointed out. “I mean, they blew up the aliens base and instantly won. It's so unrealistic!”
“Wanna talk about huge magical rainbow power beams?” Sunset smirked.
“Besides, that's what happened with the sirens,” Rainbow broke in.
“Oh, oh! You should have been there!” Pinkie Pie almost shouted. “We were like 'waaa!' but they were like 'wooo!' and we fell like 'baaam!' and then Sunset picked up the microphone and was liike 'woo-hoo!' and there was a giant horse in the sky and then rainbow like 'waaam!' and then-” she was yelling and waving her arms chaotically in the air until Applejack covered her mouth with her hand.
For a moment everybody was looking at the pink girl.
“Anyway...” Sunset moved her eyes to Twilight. “You know the story. The sirens' neckleces shattered and they ran away.”
Twilight reluctantly admitted they their real lifes often made little sense, but she didn't want to give up that easily.
“Yeah, but their power depended on it and here-"
Rarity placed a hand on her shoulder. “You might be overthinking it, dear.”
“And how did you like it, Sunset, huh?” RD asked.
“Me? I think it was pretty good. But PowerPonies are better,” the redheaded girl shrugged.
“What now?” AJ cocked her head.
“It's a comic series back where I'm from.”
“There's no way something with such a lame name is better than the Avengers,” Rainbow said.
“Oh, please,” Sunset snorted. “You wouldn't know. I'm telling you, they're the best. I have, like, at least fifty issues.”
“So let me read it and we'll talk then.”
“Um...” Sunset rubbed her arm awkwardly.
“Sunset, are you ok?” Fluttershy asked, concerned with her sudden change of emotion.
“Yeah, I'm fine. It's just... I kind of, you know, left most of my stuff... back home.”
“We can always go there-” Rainbow began, but Applejack shushed her.
“What's your problem, AJ?” the blue-skinned girl asked.
“Ah think Sunnset means the other side of the portal.”
“Oh.”
“But couldn't Princess Twilight send them over?” Pinkie asked. “It could be so much fun!”
“She's away in Yakyakistan- um, that is far away from Equestria,” Sunset added, seeing the looks on her friends' faces.
“Well, looks like it settles it. The subject is closed,” Twilight said.
“Yeah, I win,” Rainbow smiled.
“Truly, Rainbow Dash!” Rarity reprimmended her. “Do you have to?”
“What? She can't prove her point. So I win.”
“It's not my fault, though,” Sunset pointed out.
“Acctually-” Sci-Twi started, but Applejack nudged her and shook her head.
“That is a touchy subject ,” Fluttershy said quietly.
“Yeah, whatever,” Rainbow waved her hand. “It's not like some lamely named comic book from pony world could be better than the Avengers, anyway.”
Sunset narrowed her eyes. Rainbow Dash has just said her favourite comic series was lamely named, surely worse than something else without even having a chanse to properly compare them AND that originating from Equestria made it so.
She was already getting competitive when RD questioned the awsomness of THE most awsome comic series Equestria has ever known. But this...
“Are you implying pony comics are worse?” she asked.
“Isn't it obvious?” the raibow-haired girl replied.
“Rainbow Dash!” Rarity exclaimed.
“That's... not nice,” Fluttershy whispered.
“Take that back,” Sunset said through gritted teeth.
“When you prove me wrong,” RD smirked.
“And how is she gonna do that?” Applejack asked sceptically.
“I dare you to go to Equestria and bring the comic here!” Rainbow pointed her finger at Sunset.
Sunset unconsciously clenched her hand into a fist.
“Sunset Shimmer...” Rarity warned.
The unicorn-turned-human was always a one to act on emotion. And now her pride as a PowerPonies' fan and Equestria's citizen in general was seariously hurt.
“Challange accepted!” she spat out.
“Ouuuu!” Pinkie Pie squealed.
“Let's go to the statue,” Sunset said and not waiting for the rest she headed to the CHS, leaving them behind.
The five girls (ok, maybe four, because Pinkie doesn't count) who have been watching the whole thing, used it to surround Rainbow, intending on giving her a piece of their minds.
“What was that s'posed to be?” Applejack demanded angrily. The others voiced their agreement with her.
“What?” RD looked at them. “That girl has to get over the 'oh-I-can-never-go-back' thing and she needs motivation. What better than her pride?”
The others had to agree, non the less surprised with the brilliant idea.
“Yeah, I know I'm awsome,” Rainbow smiled smugly. “You guys actually didn't believe I would say that, right?”
“Oh, we did, alright. T'was comin' from you, after all,” AJ smirked.
“Yeah, I knew th... wait, WHAT?!” the blue-skinned girl exclaimed and ran after Applejack, who was already ahead of her.
“Yeah, speaking of pride,” Twilight muttered under her breath, as they all followed to witness Sunset going back to Equestria for the first time since her reformation.
*************
The shimmering sun had already set and the first stars glimmered in the last rays of twilight, like sparkly eyes watching seven girls gathered in front of the Wondercolt Statue.
Sunset gazed uncertainly at the portal, her agitation having had worn off as quickly as it came, replaced by worry.
“You know you don't have to do this, right?” Fluttershy asked quietly, always the one to sense when somebody felt bad.
“No. I do. I have to do this,” the flame-haired girl shook her head. Before her friend could object she added smiling slightly. “ I'm not stupid. I know why Dash said what she said. I'm still offended, mind you,” she pointed at Rainbow. “But I think that in this one thing she's right.”
“I'm not sure if you can come through. You said it yourself, the other me is away. How are you going to open the portal,” Twilight pointed out.
“Oh.”
“Yeah.”
“Right.”
Only then all of of the girls seemed to notice the obvious obstacle. In the last moment, like they always did when something grand was about to happen.
“Um... I guess I forgot about that,” Sunset admitted and there fell an awkward silence.
But then Pinkie Pie broke it.
“Pfff! That's an easy one!” out of nowhere she appeared behind Sunset, taking her journal she used to write to Princess Twilight out of her bag. “We just have to take the magic from here-” she motioned the thick tome, “to here!” she pointed at the statue. “From there to here, here to there, there to here heretotheretheretoherehereto-” the stream of her words was cut off as Applejack covered her mouth with her hand. She seemed to have to do that a lot, recently.
“Yeah, we get the idea,” she said.
“But how do you wanna do that?” Rainbow asked.
Pinkie Pie without a word took the journal, held it in front of the statue base and took a mighty, overexagerated swing.
“What are you-” Sunset asked worriedly but a loud THUD didn't let her finish, as Pinkie punched the book with full force, sending it slamming into the hard stone surface.
The cries of protest from her friends died down almost immidiately. They all stared at the water-like magical structure of the now open portal.
“Gah...” was all the girl with flame-like hair could articulate. She shook her head violently. “How... how did you do that?!”
“Magic of randomness, duh!” the pink girl shrugged.
Nobody dared to question that, because they were too afraid their minds might explode.
“So, what are we waiting for?” Rainbow Dash brought everyone back to earth from the depths of their thoughts.
“Oh, right. See ya!” was all Sunset said before jumping into the portal.
“DON'T YOU EVER FORGET US!” Pinkie Pie yelled after her, poking her head through the magical surface.
“She'll be back in a few hours at most,” Applejack patted her on the arm but the pink girl didn't react.
“Um... Pinkie?” Fluttershy called quietly.
They had to wait a while until the reply came.
“Eee... I think I'm stuck. He he...” she laughed awkwardly.
Rarity couldn't help but face palm.
It took the combined efforts of Applejack, Rainbow Dash and Fluttershy to pull her out, after what they all fell on Twilight and tripped the white-skinned girl along the way.
It was too much even for her and her good manners. She just had to groan loudly.
“Was that a dinosaur?” Pinkie asked from the top of the pile reciveing yet another, even louder groan.
“No, I know! Spike the Dragon!”
“What?!” Sci-Twi exclaimed.
Celestia is not surprised
Sunset's muzzle hit the stone/crystal/whatever it's made of floor as she fell head-first through the mirror. No matter how many times she travelled between dimensions it never got better. If anything, it felt worse every single time. She shook herself like a dog to get rid of the impression Pinkie Pie's calling after her. The portal with all its swirling colors and shape changing must have been getting to her head.
She cautiously rose to all four hooves, before remembering that walking on four limbs didn't need as much adjusting as walking on two, due to it's obviously better stability. The now unicorn looked around curiously. The room she found herself in was round and made enirely of what looked like crystals in many weirdly-matched colors, unreasonably sparkly and ridiculously decorated.
“Wow, Twilight. Overdoing it a little, aren't you?” she muttered to herself.
Apart from the magical mirror built into a complicated machinery and one set of doors, there was nothig significant in there.
Sunset walked over to the door and attempted to open it, but it was closed. A simple opening spell didn't work either. Upon further examination the orange mare found out the entrance was sealed with so much advanced alicorn magic it was almost impossible to break through.
'Not sure if I should be disapointed I can't exit or proud that this time she actually closed the door' Sunset thought sarcastically, remembering the tale of how one Starlight Glimmer broke into the castle because Twilight went to Canterlot without closing the door.
But jokes aside, the closed way out remained a problem.
Sunset took a while to concider her options. There weren't many.
The most reasonable would be to go back and wait this few days or weeks until Twilight came back, but she couldn't do it- she wanted to win this bet, as ridiculous of a motivation as it was.
After a lot of unproductive thinging she got so frustrated she just had to kick something. As fate had it, Twilight's portal-opening machine stood in the way.
Something broke and all of the sudden a frightening amount of magic shot out of the portal. The swirling rainbow light hit the door to the room with a mighty roar, awaking a gigant cloud of dust and smoke and a havy rain of crystal shreds. All Sunset Shimmer could do was hide her head in her hooves cowering on the ground and wait.
It took her a lot of effort to bring herself to peak from between her forelegs at the outcome of the explosion.
Her jaw dropped and she stared wide-eyed.
The floor was covered in shattered glass, crystals and dust. The drapes that hung on the walls burned down to ashes. A cloud of smoke was slowly settling down around a giant hole in the wall surrounded by a complicated pattern of cracks and smaller holes.
Sunset slowly got up and manouvering between sharp shreds walked over to the opening. There, on the floor, layed a piece of wall attached to the intact door.
“That must have been one heck of a spell Twilight used,” she said abnormaly loud, because her ears were still ringing from the sound of the magical beam.
Now she just had to get to Canterlot, grab her comic books from the castle, which meant a break-in coming and return here. As simple as that.
*************
The sun in equestria has already set and the moon rose high into the night sky. Sunset took a while to gape at the marvelous stars of her homeworld, so much brighter than on the other side of the portal.
She decided to try and teleport inside the castle, straight to the room that used to be hers and the back out. She new from Twilight that Princess Celestia kept the door closed for years, so she hoped her things were still there.
In a flash of cerulean light she was gone from the castle gardens (don't ask how she got there, Celestia's former students have their methods) and appeared inside the building.
*************
She immidiately knew something went wrong. It most certainly wasn't her room. The air was filled with the sound of running water from behind the side door. The walls were lined up with book cases, in the middle of a soft carped stood a giant bed, too big for any pony of normal size. In the corner was a perch for a giant bird, with said bird sitting on it- a phoenix.
“Hello, Filomina,” Sunset recognised the bird and was regonised too. “Sorry, I don't have any treats for you today,” she said.
Then she understood what she just did, everything clicked into place and she paniced.
She landed in her former mentor's room, a few doors up the corridor from her own.
She jumped to the door, her mind set on running away while she still could. But her magic was strained from teleportation after a long time of laying dormant and she didn't manage to open the door.
BANG! She slammed her head into the wooden surface.
The sound of flowing water stopped.
“Luna? Is that you? I told you I haven't been using a rubber duck for years. If you're still trying to take a photo of me with it I swear...”
Suddenly Sunset remembered how she used to practice her animal-scare-off spell on Celestia's rubber ducks. Somehow they always ended up chewed to shreds.
The door opened, revealing none other than the Princess of the Sun herself. She didn't look very regal at the moment, though.
Her coat was completely wet, her mane and tail hung limply, shampoo dripping from them. Her eyes had dark rings around them, clearly implying the princess had had a bad day and was simply exhausted.
Sunset bliked. She might have even laughed at the sight had she not been so terrified.
Princess Celestia, expecting her sister, at first serarched above Sunset's head. Then she looked down.
“Oh. Hello, Sunset,” she said as if she had seen the orange unicorn just the day before.
“I'm sorry about the rubber ducks,” was all the girl could say.
“Don't worry. I didn't need them anyway,” the princess smiled.
Sunset struggled to keep a straight face as she caught a glmpse of one of said ducks on the floor behind the co-ruler of Equestria.
“How has your day been? Have you learnt anything interesting?” the white alicorn continued.
“Oh, it was fine. Today I've learnt an important lesson on the magic of randomness,” Sunset replied seariously. “And accidents with magical portals.”
“I didn't know they teach that at school,” Celestia raised an eyebrow.
“It was... an additional subject to improvised planning,” Sunset forced herself to smile.
“Oh, alright then,” the princess said. It was obvious she didn't want to think to much at the moment. “Is there any particular reason why you came here?”
The unicorn rubbed the back of her head awkwardly. “Um... I kind of missed my room while teleporting.”
“Sunset Shimmer, what did I tell you about teleportation?” Celestia frowned.
“Do not attempt inside,” Sunset said as one can say only something they repeated countles times. “I know.”
The Sun Princess was still looking at her.
“I'm sorry?” Sunset tried.
“Eh, when will the day come you will actually listen to me,” the alicorn sighed, accompaning her former student to the door, clearly unaware of the trail of water and shampoo she left behind.
Sunset was already standing outside the door when she felt there's one thing that's been missing in all of her partings with her mentor.
Without any warning she threw her forelegs around Celestia'd neck.
After a while the surprised alicorn returned the hug. “I love you too, my little Sunset.”
All too soon Sunset Shimmer found herself standing alone in the corridor, her heart stinging with pain of unknown source. She had no idea what just happened, but at the same time she knew it should have happened long ago.
Then she remembered why she came there. She shrugged and went for the comic books.
************
On the other side of the door Princess Celestia returned to her bathroom, so exhausted she tripped along the way as the whole world went spinning. She shook her head. She desperately needed some rest. Then she remembered what just happened.
“Was that... No, impossible. It couldn't have been her. I really need more sleep. Maybe I should ask Luna if she could rise the sun tommorow when she co...” she didn't finish because her hoof instead of the floor found a rubber duck and The Princess of the Sun fell to the ground.
There was a bright flash, a sound of camera and a loud giggle, followed by the sound of hoofsteps of somepony running on the marble floor.
“Luna?” Celestia asked then realised what that meant. “LUNA! COME BACK HERE!” she yelled.
