Unfortunately for the crusaders, their plans for the day were ruined since their rules forbade crusading without any one member and Scootaloo had to leave for Cloudsdale with her family.
Obviously, Nightmare Belle couldn't let go of such an opportunity and convinced Sweetie to start researching the spell.
And so, early in the morning, the two remaining crusaders arrived in Canterlot.
"Not that Canterlot isn't a nice place," started Apple Bloom cautiously looking around as they left the train station "But was it really necessary to come here?"
"Why?" asked Sweetie pausing to look at her friend with a crocked eyebrow.
"Ya know, fer all that incident with Nightmare Moon..." said the yellow filly dragging a hoof on the ground.
At those words, Sweetie stiffened visibly.
"Now Bloom, it's not like we're wanted criminals or something..." she said putting on an awkward smile, yet she casted side glances at every reflecting surface trying to spot her dark counterpart.
Luckily for her, Nightmare Belle was nowhere to be seen.
"Ah hope yer right," said the other as they paced towards the library "Ah'd like to avoid being beaten by the guards again."
"I'm sure the princesses won't let them," said Sweetie putting a comforting hoof on her friend's shoulder.
Luckily for them, nopony seemed to hold a grudge towards neither of the two and so they reached Celestia's School for Gifted Unicorns and the annexed library.
It was huge, with the high shelves full of leather-bound book that towered over the two fillies. The magic particles saturated the air to the point that even Apple Bloom could feel it.
Having no clue on where to find the clone spell, they started searching for the librarian instead.
Their hooves clicked on the white marble floor breaking the thick silence that reigned in the deserted library, making the two of them feel almost unwelcome, like thieves in an ancient tomb, only tidier.
Soon, a third sound of hoofsteps started resonating in the not-so-empty halls, signaling that somepony was coming their way.
From behind a bookshelf appeared an elderly looking unicorn mare with acid green coat and bright orange mane.
She regarded the two fillies (and Apple Bloom in particular) with a skeptical expression from behind her thick glasses before asking: "Can I help you?"
"Yes," said Sweetie offering a cheerful smile "We were searching for Mirror Sheen's clone spell."
"Mirror Sheen's work is very advanced magic," stated the librarian "May I ask why you're searching it?"
"For research, of course."
"Search 'Through the Mirror' in the antique section, third row on the right," said the mare pointing a hoof "Please, be careful with it, for it is a very old text."
"Will do, thank you very much."
"Ah don't like the way she looked at me," whispered Apple Bloom when they reached the antique section.
"You have to admit that not many earth ponies wander a spell book filled library," chuckled Sweetie "Let's find that book now."
It took them around a hour, but they eventually found 'Through the Mirror'. It was bound in white leather and the title engraved in polished silver, with a circular piece of mirror enchased in the cover.
Turning the yellowed pages with care, Sweetie searched for the interesting parts (signaled her by a tingling sensation in the back of her mind), which Apple Bloom copied on a notebook. Writing out two spells and some useful notes took them another hour and half, so they were out of the library by lunch time.
---
Princess Celestia sat on her throne, a poorly masked bored expression plastered on her face, as two unicorn stallions bickered in front of her about a land tax or something.
Luckily for her, a page rushed to her side to tell her that Arcane Mark was asking to be received.
"Arcane Mark the librarian?" asked Celestia feigning not to notice the two stallion's outraged expressions at being interrupted.
The page nodded.
"My good sirs, I beg your pardon but a matter of the outmost importance has arised and demands my attention," said the alicorn in a well faked anguished tone "You'll have to excuse me now."
And with that, she left the room following the page.
The two nobleponies, not knowing what else to do, resumed their discussion.
The princess met the librarian on one of the balconies that opened on the castle grounds, a place as nice as private.
"I beg your pardon for the intrusion, your majesty," said Arcane Mark bowing deeply.
"No harm done, don't worry," assured Celestia in her usual motherly tone "Now, I suppose you're here for that reason."
"Yes, your majesty," answered the green mare without rising her head "Two fillies responding the description came to the library less than an hour ago, searching for Mirror Sheen's clone spell."
"Ah, the clone spell, a wonderful way to skip the day court when my nephew comes!" said Celestia with a chuckle "I even went back in time just to thank Mirror Sheen..."
"Do you think it may be a problem?"
"Oh, I seriously doubt that," said Celestia waving a dismissive hoof "Anyway, I'd like to see the texts they examined, just for being sure."
---
After having had lunch at Pony Joe's shop, the two fillies took the train back to Ponyville.
"You did well making your friend write the spells out for you," said her reflection in the window as her color scheme darkened "She has a much neater calligraphy than you."
She was taking a risk, even if the carriage was empty and Apple Bloom asleep, but she had to make sure of the filly's capabilities.
"Yeah, I kn-" started saying Sweetie before realizing she had just been insulted and shouted "HEY!"
"Don't shout, or you'll wake your friend!" snarled Nightmare Belle before asking in a calmer tone "How much did you understand of the clone spell?"
"Nothing, more or less..." answered the white filly in a dejected tone "The librarian did say it was advanced magic..."
"Luckily for us, I can help you with that," sighed the other massaging her temples.
"Do you think I can cast it? Twilight says I'm promising but..."
"I hope you can," said the dark filly "Otherwise we'll be together for a long time, and I really don't like the idea."
"Is being with me that bad?" asked Sweetie in a dejected tone as she lowered her frame.
Nightmare Belle was taken aback by the filly's reaction: she was a parasite that used her to stay alive and to create a new body, and yet, the small unicorn looked hurt from her words.
"I didn't mean..." said the nightmare "Look Sweetie, it's only that for something like me having my own body is like a dream..."
"I'm not sure I can understand what it means not to have a body..." said the white filly with a pensive expression "I think I misinterpreted your words, sorry."
"Sweetie, who are ya talkin' to?" asked Apple Bloom.
Sweetie Belle turned to look at her friend, only to see that she hadn't opened her eyes.
"Uhm, nopony Bloom, don't worry."
When she turned again to regard Nightmare, she found only her reflection staring back at her.