The hallways of Canterlot Castle were bathed in the light of the evening sun bearing down from across the horizon. Massive windows that lined the side passages of the great castle filtered the rays to a more delicate, softer tone, allowing the viewer to peer out into the rest of the magnificent city below without being blinded.

Princess Celestia would have taken a moment, like on so many other days, to appreciate the view, but she was far too frustrated and walking far too fast.

"Sister, I must insist that you listen to-" a voice piped up from behind her, but she did not turn to look.

"Luna, please," Celestia cut her off. "You've said it yourself, she is still far too weak to pose an actual threat."

"Yes but..." The footsteps behind her stopped.

Celestia was tempted to continue walking, but with a tired sigh, she stopped and turned around. "But...?"

"Tia, this is Nightmare Moon we speak of. She is not a mare to be underestimated." Luna pleaded.

"I know, Luna. I know..." Celestia sighed wearily. "I just...I doubt that she could breach the castle, or our own defenses."

Memories of being blindsided by Nightmare Moon on the day of Luna's return from exile flooded Celestia's thoughts.

"Not this time, at least," she continued, watching a contingent from the Lunar Guard patrol the courtyards below.

"But-" Luna started.

"We will not be having this argument, Luna." Celestia stood firm.

Luna's neutral features slowly developed into a small scowl. Crossing her arms, she looked away and huffed.

"Fine."

Celestia's stance softened.

"Listen," she said in a whisper, extending her wings to engulf the smaller mare in a cocoon of warmth. "We'll be fine, alright? If she comes...we'll be ready for her. Every one of us."

Luna was caught by surprise, and her body stiffened for a moment before melting into her sister's embrace. Celestia had a point. With Luna's return, and the subsequent return of the Thestrals to the ranks of the royal guard, Canterlot Castle had become a bastion of security around the clock.

"I promise, Luna."

Her efforts rewarded her with a ghost of a smile from the night princess, and that was more than enough for her.

"Hm...I suppose we will. We..." Luna paused. "...I...will keep you safe as well, sister."

"Mirabilis es." She pecked her on the forehead. "Good night, Luna."

"Good night, sister."

With that, Luna turned in the opposite direction to conduct her nightly court, and Celestia continued on her way.

Princess Celestia sighed as she finally set the sun below the horizon. The streams of vibrant gold that entered through the balcony windows of her chambers dimmed and were replaced with beautiful silver hues of light that signaled the arrival of the moon. She had practically slumped over the moment her doors shut, feeling the weight on her shoulders lessen. Now, in the privacy of her chambers, she was free to do whatever the hay she wanted, which was exactly what she did.

She tugged on a cleverly placed string on the inside of her lavender dress, causing the silken work of art to gracefully slide off her curvaceous body and onto the floor in a heap. Next was a velvet corset that was practically ripped off in frustration, followed by a contented moan as her ample breasts popped free from their restraints.

Kicking off her high heels, she finished with a giggle as she slid off a daring strip of black fabric that was wedged between her sumptuous ass and her toned legs, wondering how the aristocracy would have reacted if they ever found out that their infallible Princess wore thongs to court.

By the time she had stripped off her garments and slipped on a pink lace nightie, the weight of the day truly began to take its toll on her. A mundane day, at best, but the workload had been steadily increasing as the holiday season neared.

After sitting through six grueling presentations and heated discussions on tax reforms, budget allocation and a debate on the need for more confectioneries for next month’s upcoming gala (Pinkie was there too!), saying she was exhausted was an understatement.

Her large, four-post bed beckoned to her as she removed her crown and necklace and levitated the golden regalia onto a stand. Sliding under the purple silk sheets, she let out another barely suppressed moan when she fell against the plush pillows and let her body yield itself to the soft, heavenly bedding. After a particularly loud yawn and stretch, she settled in, and drifted off into slumber…

It was an absolutely delightful two seconds of well-deserved rest before she was jolted awake. She opened her eyes in shock, but she was not greeted with the usual sight of the tastefully painted picture of Twilight Sparkle in a...questionable...pose that was stapled to the roof of her bed canopy. Rather, and rather remarkably, she found herself in an upright position, standing in the middle of nowhere.

There was darkness all around her, save for a bright light from above that was directly on her. She regained control of her body movements, moving her head from side to side and clenching her fists as a quick check.

She thought back…no, she had just turned in for the night, and no one had triggered her magical ward which would have woken her up the moment somepony stepped into her chambers. The other ward on her door would have been overridden itself and unlocked with a loud, audible bang, and her two door guards would have burst in within seconds.

There was no wind, no scent or sound, just an empty expanse of darkness beyond the small patch of light she was standing in.

Celestia presumed that she was in the dreamscape. It was a gut feeling, but she placed a lot of trust in her sixth sense. Having been dreaming for millennia, she had come to form a connection with the dreamscape just as much as her sister Luna had, despite her weaker affinity towards the ever changing dimension. That way, she could walk the world just as her sister could, but not shape it precisely to her desires.

But, she could think of something, and little effort on her part would set the dream in motion while the rest would come as a surprise.

Time for some fun.

She closed her eyes and stuck her tongue out the side of her lips in concentration, imagining herself in her throne room. She imagined herself seated comfortably on her plush, royal throne, free of responsibilities and the ever-scornful gaze of her courtiers. For that matter, she was also free of clothes.

She was stretching back lazily; her head against an overstuffed pillow, and her legs dangling lazily on the armrest of her throne. To her sides, bare chested stallions from her own personal guard waved large palm leaves and fed her cake, while in the center of her throne stood the centerpiece of her dream. Twilight Sparkle, all dressed up in frilly lingerie, danced next to a long, metal pole. Gyrating her hips to the slow beat of music played in the background, she shot Celestia a saucy grin as she pressed her crotch against the pole.

"Does this please you, Princess?"

Oh, yes.

With a delightful shiver, Celestia opened her eyes with a gleeful smile to greet her despicably naughty student, but her grin died the moment she was met with the same empty expanse of void.

It was then she took note of that the darkness surrounding her. Now, it seemed unnatural, and foreboding. It swirled with an odd movement that was disturbingly visceral, and it radiated an aura of malice. She had dreamed of darkness before, forlorn nightmares that haunted her as a filly, but certainly nothing like this.

She felt a chill run down her spine, and she realized just how terrified she was. There was one, and only one pony she would call on in a situation like this.

At last, she could resist no longer.

“Luna?!” She cried out into the darkness.

There was a brief pause, but then the world around her shifted in response. The inky blackness parted like a veil of water, moving aside and letting in much needed light. A loud noise echoed in the distance, like a heavy door slamming shut, and the telltale tap-tap-tap of slow footsteps made it clear that she was not necessarily alone.

The retreating darkness allows Celestia to see a little more, and in particular, a figure that was approaching her in the distance. She squinted to see properly, but once her vision adjusted to the sudden change in brightness, she could make out the pony that had stopped just a few feet away from her.

A creeping dread washed over her, sending a terrified shiver through her entire body. She gasped.

“Oh Goddess…”

“Why, hello, Celestia." Her voice was smooth and sweet like fine aged honey, albeit laced with venom.

Celestia's eyes went as wide as dinner plates and the mouth hung open, desperately trying to form a response as she watched the other mare begin to walk in a predatory circle around her.

"N...N...nuh...n..." Celestia blurted.

"Cat got your tongue?" The mare tsked. "I'm disappointed. You're usually quite the orator."

"Nightmare...Moon...." Celestia spoke in a barely audible tone.

The mare responded with a smirk. There stood Nightmare Moon, in all her terrifying and sensual glory. Celestia recognized her by her wispy, starry mane that drifted in an ethereal breeze, along with her draconic arctic-blue eyes, accented by the purple eye-shadow she used.

She had forgone her iconic crystalline armor and helmet completely, opting for a shimmering black dress that hugged her curves and taut, slender body. The dress parted along her hips to display her firm thighs and long legs, and similarly, a narrow opening just above the chest displayed her prominent cleavage through the near transparent material.

It sent a rather daunting message to Celestia that she wasn't the only sex goddess in Equestria.

"We bid thee welcome to thine worst nightmare." Nightmare quipped, mockingly curtseying to the sun princess and snapping her out of her stupor.

“ W..What do you want? W have you done with Luna?! If you hurt her again I swear-...”

“I've not hurt my beloved Luna," Nightmare sneered. "So fret not."

Celestia's eyes only narrowed in response. "Then why am I here?"

"There is the matter of something that needs to be settled between you and I."

"Ahh, petty revenge, now is it?" Celestia growled. Her horn began to glow as she charged her magic. "You forget, I have spells that can let me leave the dreamscape as I please. I'll be awake before..."

There was a pop, and a little fizzle of sparks, signaling a failed spell.

“Try again.” Nightmare smirked as she urged Celestia on.

“I…” Celestia closed her eyes and focused on her magic, only to find it completely unresponsive to her.

Crap.

Nightmare laughed wickedly. “Is there something wrong, Celestia?”

"What..did...you...do...?" She replied through clenched teeth.

"Foal," Nightmare gestured around her with her hand, eventually pointing upwards above her. "There is no magic in this domain. Only a ward."

True enough, a massive round glyph glowed green and purple, surrounded by rings of light and markings written in ancient Equestrian. Celestia blinked, flabbergasted as to how she had missed it in the first place.

Celestia snarled, her patience wearing thin. “Then what is the condition to break it?”

Nightmare still wore her grin. “Simple…This dreamscape will collapse upon itself and you will be freed if...you defeat me mare-to-mare.”

Celestia hmphed haughtily.

“What? Afraid to get physical with me in the real world, Nightmare?” Celestia taunted.

"Oh, perish the thought, Celestia. You are in my domain now, and..." Nightmare shot a sideways glance at her nemesis. "I can sense your fear."

It was true, to say the least. Celestia's legs trembled slightly, and she struggled to maintain the confidence in her posture and voice. She wondered how long she could last before her body did something she would regret.

"Now...should you emerge victorious, you will awaken." Nightmare turned to face Celestia directly. With a wicked grin, she continued. "Should you be defeated, you will remain here for all eternity, and not even Luna will be able to break you free."

“…No….” Celestia’s eyes widened in fear.

Nightmare began a slow and steady saunter towards her prey.

“Yes. There is no other choice, Celestia! Fight me, or be tormented forever!”

The dark mare had reached within striking distance of Celestia. One more step, and Celestia gasped the moment Nightmare pressed herself against her counterpart in a display of pride and confidence. Bountiful breasts were first to push against each other, and in the dangerously close proximity they were in, both mares found themselves getting moist as they strained against one another.

"Your choice." Nightmare whispered.

Celestia stifled a moan, but in a show of defiance, balled her fists angrily. There was no way in Tartarus that little moon whorse was going to trap her in a damned coma.

"I accept your challenge, Nightmare Moon!” There was too much on the line here to back down, both for Equestria and her own pride.

Nightmare grinned, and she licked one of her razor sharp canines in anticipation.

"Good."

She snapped her fingers, and the world went dark once again.

Author's Note

Written on a dare, posted on a bet. (If you like the story, I win a week of lunch!)

The fight begins in the next chapter.
Bring popcorn.