...And there, standing motionless behind them, was THE SLENDERPONY.
In a quick frenzy of action, Applejack ran for the door, Rarity close behind, as Rainbow Dash grabbed Fluttershy and flew around it and out of the window, while Twilight teleported to the front of the library.

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And through all of this, the tall pony with no face simply stood there. But as the six ponies fled, he reached upwards... and in a single swift motion, yanked off his head, revealing a benevolent and highly amused face and a flowing rainbow mane.
Princess Celestia walked slowly towards the monitor, opening a document in Minisoft Letter, to write a brief message to her student.

Dear Twilight Sparkle,

I hope that this message finds you in good health. It was slightly disconcerting when you ran away from me, is everything alright? Please reply swiftly, because I had hoped to show you my costume for this year's Nightmare Night celebration, as I am planning to attend the festivities in Ponyville.

Sincerely,
Princess Celestia

And with that, the Princess of the sun leapt out of the window and flew off towards Canterlot.

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As she and her friends regrouped in front of the library, Twilight stepped towards the front door. "I'm going in. If this thing is a danger to Equestria, it's my duty to stop it!"
Applejack galloped up to her friend. "Wait up, there. We're all in this, y'know. Right?" she said, looking back at the other four.
"Yeah!" Pinkie exclaimed, bouncing towards Applejack. "I can't let you fight a superduperevilscary monster alone!"
"Yeah, lemme at him!" Rainbow yelled, making boxing motions. "I'll show that faceless monster not to mess with my friends!"
"Well, I suppose we can't do this any other way, can we?" Rarity said, brushing her hair from her face.
Fluttershy walked slowly, but resolutely towards the front door. "I...I can't let him hurt anypony... but I think we should try something other than fighting first."
All six ponies slowly entered the dark, book filled tree, silently climbing the staircase.

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"Where IS he!?" Rainbow yelled, triple-checking the room. "It's not like he can.... oh, wait..."
Hmm... I suppose he.... Hey, what's this?" Twilight said, suddenly noticing the message on the computer screen.
"What?" Spike mumbled sleepily, having been woken up to aid in the search.
"Everypony, come over here, I've found some kind of letter!" Twilight exclaimed, thinking that she was finally on to something. They gathered around the bright screen and began to read, their cheeks growing a shade of red deeper with each sentence. As they finished, Twilight turned to her assistant. "Umm... could you take a letter?"
Spike grinned, amused by the thought of Twilight and friends accidentally fleeing from Celestia. "Sure!"

Dear Princess Celestia,

I am doing well, and I hope that you are as well. I believe that there was a misunderstanding tonight. Me and my friends happened to be playing the game that you suggested to us, and we were unprepared for your visit, thus we were not expecting you to come in costume. It would be wonderful if we could arrange a time for you to come to the Nightmare Night celebration this year.

P.S. The costume was very... realistic.

Your student,
Twilight Sparkle