It was a mean but funny prank, Tia!
Nightmare Night; the one night where ponies could dress up in their costumes, getting free candy and have a wonderful time with one another while not getting arrested by the guards for 'scaring the populace' for wearing scary costumes that sometimes realistically looked like monsters (in a few Nightmare Night events, actual monsters attended the festival sometimes! Thankfully, they were just there to have fun, too). Of course, like all things, there is more to it, and that includes pranks. It was a prime night for pranks, and Nightmare Moon (and company) was convinced that they were about to pull the ultimate prank on Fluttershy, who also happened to be the easiest one to prank, thanks to their intelligence network (which was stalking Fluttershy for a few hours each day, alternated between the conspirators, for the past two weeks).
Fluttershy didn't celebrate Nightmare Night, and while indeed she was Fley Truths, even she would be at the very least surprised about what was coming to her; she didn't take very well to pranks. Raising a hoof, Nightmare Moon knocked on the door, awaiting for Fluttershy to open the door.
Slowly, said door creaked open slightly, revealing Fluttershy looking out cautiously. "Erm...I-I don't celebrate Nightmare N-Night..."
Sensing that Fluttershy would say something like this, Nightmare Moon merely smiled at the pegasus. "No worry, I have not come to ask you for candy! I have come to ask you for a place to reside in temporarily!"
"Um. Okay." The door slid open, and Fluttershy stepped aside, allowing Nightmare Moon inside. "I guess you can stay and rest up for a bit, Moonie."
"Thank you, Fluttershy." Nightmare Moon sat down on a rather comfy sofa, satisfied that she was able to gain entry into Fluttershy's domain. "It is rather exhausting out there, and the festival gets too loud for me to comfortably rest in any of your friend's homes! Rarity suggested that I rest in your home, where it is far from the celebration."
"When you put it that way, it does make sense. "Fluttershy sat down next to Nightmare Moon, smiling slightly. "Do you need any refreshments?"
With a slow shake of her head and a wave, Nightmare Moon refused. "No, I merely require my royal rump to rest!"
"Okay. Well, I'll be-"
The sound of knocking upon Fluttershy's door took both their attentions. "Erm. Excuse me, I'll get the door." Fluttershy got off the sofa, and slowly walked on over to her door, wondering who it could be. She cracked the door open slightly, just enough to whisper out. "Hello?"
"Greetings, Fluttershy!"
Confused upon hearing the voice, Fluttershy opened the door to see Nightmare Moon, smiling at her. "I've come to raid your supply of candy! Trick or treat!"
"I...Uh...Didn't you...Already come inside?"
"What are you talking about? I have just arrived here!"
Fluttershy turned her head around, and sure enough, Nightmare Moon was still sitting there, at the sofa. Unsure of what was happening, she pulled her door open, and stepped aside. I-It can't truly be another Nightmare Moon. It's probably Twilight; I mean, she could probably use her magic to look like an alicorn! She had weird taste, though; I'd prefer if she changed into Cadence. Pink is better than black...No offense. It would also look better on her. Still, a very convicing costume! "Erm...I don't usually celebrate Nightmare Night, but...I do have some candy upstairs. Make yourself welcome, Nightmare Moon." Fluttershy slowly backed away from the second Nightmare Moon, before walking up her stairs a little quicker than anypony should be when going up a staircase.
The second Nightmare Moon giggled, and walked over to the first Nightmare Moon, winking at her once Fluttershy had gone upstairs. "Flutters totally bought it."
With a grin, the first Nightmare Moon winked back. "This idea was genius, Chrys!"
"I know, right? And you have such a flattering figure; it goes well with this awesome armor you wear."
Blushing slightly from the comment, Nightmare Moon turned her head away slightly, and looked up at the staircase. "Thanks, Chrys. I can't wait until Luna knocks! Oh, shh, shh, Flutters is coming down!"
Just then, Fluttershy slowly walked down the stairs, with a small bag of candy that she kept for herself whenever she needed her sweet cravings sated, but she figured she could always buy more candy later. "I have some candy for you, err...Second Nightmare Moon. I-"
Knocking was heard again at her door for the third time, interrupting her, and Fluttershy slowly turned her head around to the door, dreading what she might find on the other side. She did, however, give the second Nightmare Moon her candy, first, before she walked over to her door, opening it slowly,hoping it wasn't what she thought it was going to be. "H-Hello?"
Fortunately, instead of a third Nightmare Moon, it was simply Princess Celestia, who had decided to go for a slightly more youthful look, keeping her mane pink and donning her golden armor (which, while scary to think about Celestia fitting into a suit of golden armor, was offset by the princess' friendly demeanor). "Hello, Fluttershy. I understand that you don't celebrate Nightmare Night, but I figured it wouldn't be too long of a shot to ask if you have any candy laid out for those that do make the trek to your home."
"Oh...It's no trouble at all, Princess Celestia! I do have some candy, in fact." Fluttershy tried very hard not to look behind her, over at the two Nightmare Moon's sitting over at the sofa, and brought out her bag of candy, giving Celestia a few pieces of chocolate. "Happy N-Nightmare Night, Princess!"
"Happy Nightmare Night, Fluttershy." Celestia smiled warmly at Fluttershy, then stepped aside. "C'mon, Lulu, don't be shy!"
...Uh oh. With wide eyes, Fluttershy watched as Luna stepped up in her view, looking just like Nightmare Moon. At this point, her brain couldn't comprehend the ability to think of the fact that there were indeed three Nightmare Moon's in close proximity with each other, and so, she promptly fainted on the floor.
Concerned, Celestia barged into Fluttershy's home (just barely being able to fit through the door, no thanks to her armor and rump), and checked her condition. "She's still breathing...I think she got scared of you, Lulu. I told you dressing up as Nightmare Moon wasn't a good idea!"
"Apparently not, because I see two more of me over there." Luna spoke lamely, pointing over at Nightmare Moon and Chrysalis.
Celestia looked up at her sister, and then followed her hoof over to see the other two alicorns. I don't think I even need to ask what happened. Ugh, Chrys, Moony...You two certainly like to push the envelope! "The only thing I can ask is 'why'?"
Chrysalis spoke up, in a very enthusiastic voice. "It was a mean but funny prank, Tia!"
A new voice not belonging to either of the three piped up. "It was mean? It didn't seem like it!"
The three alicorns turned around to see a fourth Nightmare Moon, who happened to be Pinkie Pie; one could easily tell because her hair was still wild and untamed. "Whoa! I seriously thought I was the only pony going as Nightmare Moon tonight, I didn't think you all would, too! Did I miss out on all the fun?!"
"You? Miss out on the fun? That's new. I must add that to my calcul..." Twilight Sparkle's voice died as she saw that there were five Nightmare Moon's, counting herself. "Well."
"Well." Celestia responded, taking a few moments to look at all of the gathered Nightmare Moons that she was surrounded by, then down at Fluttershy. "I know Fluttershy's going to be having nightmares."