Once again story by :iconvelcroheart: not me  
My faithful student Twilight Sparkle,  
Despite my worries, my dearest sister Princess Luna thoroughly enjoyed last year’s Nightmare Night. It does me good to know you helped her enjoy a festival that would’ve likely touched a sensitive nerve - I, myself, have never partaken of this celebration in fear of offending my sister, even before she was returned to me by the Elements of Harmony.
 
Since I no longer have to worry, though, I’ve decided to join in on Ponyville’s night of costumes and terror this year, and I hope you, Twilight Sparkle, will help guide me through the various events.
 
As such, I’ve had two matching costumes made, so we can roleplay our entire night away and have fun as only Luna could describe to me.
 
Please open the included box only on Nightmare Night. Don’t worry, I was assured the outfit would fit.
 
–Princess Celestia  

 
Nightmare Night had come, and Twilight Sparkle was almost prancing with excitement. Sure, it was a bit of a dissapointment that she didn’t get to wear her Ancient Literature Book costume, as she’d worked quite hard on ensuring the lettering matched the true volume, but whatever Princess Celestia’d planned, it was sure to draw the attention of quite a few ponies!
 
And that big, cardboard box had been taunting her for almost a week now, its sheer mysteriousness calling, demanding she sate her curiosity.
 
But now was finally time to unveil—
 
Some dark pegasus wings? A pegasus? Maybe Princess Luna? Maybe…
 
The helmet that came out of the box next was a dead giveaway: Nightmare Moon.
 
Twilight Sparkle was going to be dressing up as Nightmare Moon tonight. That was an.. interesting choice. What would Luna think?
 
It didn’t take too long to get into her new outfit, luckily, yet unfortunately that left her with a great deal of time to worry about the princess’ arrival.
 
 
It was almost eight o’ clock when Twilight Sparkle heard the familiar sound of teleportation, and turned around to find herself gaping at a familiar alicorn in a very unfamiliar outfit.
 
“P-princess Celestia! Chains? A collar? A leash? W-what are you going as? Ehm…” Twilight Sparkle’s brain desperately scrabbled to guess what those odd accoutriments might mean.
 
Princess Celestia just smiled, faintly, and leaned forward to whisper; “I’m going as Nightmare Moon’s thoroughly defeated alicorn slave, Twilight Sparkle. Or, should I say Mistress Nightmare Moon..?”
 
Twilight’s stunned expression made it quite easy for the princess to levitate her leash into the unicorn’s gaping mouth…
 
Mentally, Twilight Sparkle reassessed her earlier estimation: This was going to draw the attention of a lot of ponies.