Author's Note:

Yes, it happened. I have become a brony!

It was the night of the Grand Galloping Gala, an annual ball held in Canterlot, capital city of Equestria and home to their wise and benevolent ruler, Princess Celestia. And this time around, the Gala would be different, as it was attended by the princess' prize student, the unicorn pony named Twilight Sparkle, and her friends. To say that their actions caused more than one stir was an understatement.

But this story will not tell the exact events happening at the Gala, but instead will focus on one pony that did not attend the Gala… even though she was more than welcome to join. It was the younger sister of Princess Celestia, Princess Luna. Celestia had invited her to join her at the festivities, since this was the first Gala after Luna had returned to her home and sister.

Because the past thousand years, she had been imprisoned on the moon, banished there by her older sister herself…

Ever since Twilight Sparkle and her five friends had turned her back to her normal self, using the Elements of Harmony, Luna had sworn to herself that she would never allow herself to be turned back into the evil, black mare she had been, Nightmare Moon.

Instead of joining her sister and their guests in the ballroom, the banquet hall and the palace gardens, she locked herself up in the castle's library, reading one book after the other.

Luna had been feeling restless the past couple of days. After she was welcomed back in Equestria, she was just overjoyed to be back to normal. But not soon after that, she started thinking. How could she have been turned into Nightmare Moon in the first place? Was it really just her feelings of anger and jealousy that transformed her? Or… was there some other force at work? Even she did not know everything about the mysteries the moon held.

But even after days, even weeks of searching and studying, she could not find anything within the old, dusty books inside the library. There was no similar case of such a transformation in the history of ponykind before her own either.

Luna sighed to herself as she turned around to the huge bookcase behind her. There were still hundreds of books she had not touched yet. "I have to find out more," she muttered to herself. "I can't allow this to happen again…" Using her magical horn, she levitated one of the books down to the table she was standing at, opened and began reading anew. She stayed in the library until after midnight, and even after most of the guests had left. She was so focussed on her work that she did not even hear it when the stampede of animals came rampaging through the ballroom downstairs.

Hours later – the new day was already dawning – her sister softly knocked against the library door with her hoof. "Luna… are you still in there? The guests have left… Twilight and her friends have also returned home by now. Don't you want to go to bed?"

Luna was feeling tired, but she had the feeling she was getting close to finally solving the mystery. "Not yet, sister!" she shouted through the locked door. "I have to figure this out!"

"You really should take a break… can't you at least open the door? Can we talk? I'm worried about you, you know?" Celestia asked.

Luna really was glad that her sister didn't hold a grudge against her… for all she had done as Nightmare Moon, it wouldn't have surprised her… but Celestia wasn't like that. She was the most forgiving pony in Equestria and welcomed Luna back in Canterlot right after she had turned back to normal. Still, she felt… uneasy when talking to her. As if she had no right to enjoy the closeness to her sister… as if she didn't deserve it.

"I know I'm close to finding the solution, big sister," Luna shouted. "I'll be coming out as soon as I'm finished, not earlier."

She could hear Celestia's sad sigh from behind the door. "Very well… I'll be in my room. If you need anything, if you need to talk… I'll want you to know I'll be there for you, little sister. Always."

Meanwhile, in a completely different country called Gensokyo…

This morning, Yukari Yakumo awoke with a growing feeling of unease. She was unable to tell where this feeling was coming from, but it grew stronger the more she tried to figure it out. She then did what she usually did when getting such a feeling: Checking if the Hakurei border, the boundary that kept the human world separated from Gensokyo, was still intact. It was as stable as it was every other day.

However, the feeling of unease remained. It was like a horrible itch that you know about, but scratching wouldn't work, since you don't know the exact source of the itch. Whatever it was, Yukari was certain about one thing: Itches like that normally only occurred whenever there was a major 'incident' befalling Gensokyo.

Her shikigami Ran opened the door to her bedroom with a bright smile. "Rise and shine, Yukari-sama! The sun is already up in the sky! Let's meet this brand new day with a smile."

Ran really wasn't surprised to find her mistress still in bed. For those who knew her, Yukari was infamous for being a sloth. Everyone knew that she preferred to let Ran do the housework and only get in action when there was something really important happening that needed to be taken care of.

Which was why Ran was surprised when her mistress jumped out of bed faster than she had seen her move in… was it months, years? And the way she quickly got dressed made the kitsune wonder if there was something wrong with her.

"Yukari-sama… you are not sick, are you?" Ran asked as she felt her forehead. "Shall I get you a thermometer? Do you want me to prepare a hot footbath and some warm blankets?"

"Stop that, Ran," Yukari said in annoyance. "This isn't the time for jokes. Now, get me my umbrella… I need to go out. Please take care of the house while I'm gone and prepare a huge dinner for when I return in the evening, I'm probably going to be hungry."

Of course Ran sighed when she found herself once again burdened by all those tasks. "But Yukari-sama… where are you going?" she asked while she gave Yukari her umbrella.

"To the moon!" Yukari replied before creating one of her portals out of thin air. She stepped through and left a bewildered kitsune behind.

Ran blinked a couple of times. "The… moon?" she blurted out. "Cheeeeeeeeen!"

"Whats's the matter, Ran-sama?" asked the young, cute Nekomata resident of the Yakumo household and own shikigami of Ran as she popped up next to her mistress.

"Yukari-sama is acting weird again…" Ran muttered. "Please be a dear and get me some aspirin… I have the feeling I'm going to need it…"

Before heading to the moon, however, Yukari appeared in the middle of Eientei, the huge house in the middle of the bamboo forest, home to two former residents of the moon… the fugitive Lunarian Princess Kaguya Houraisan and her advisor/genius pharmacist, Eirin Yagokoro. Oh, and a countless number of rabbits, too.

Yukari appeared right next to a certain earth rabbit called Tewi, who let out a gasp of surprise. "WHAH! Who are you?"

"No time for introductions," Yukari said in a stern voice. "Your princess knows who I am… just tell her Yukari is here and needs to speak with her. Nevermind, I'll tell her myself… where is she?"

"N-now wait a minute…" Tewi stuttered. While she loved playing pranks on others, she hated being surprised out of the blue like that. "Princess Kaguya is extremely busy at the moment… as is Mistress Eirin, to be exact…"

"Yukari?" another rabbit, this one taller, with long, purple hair asked as she stepped into the corridor where Yukari had appeared just a moment ago. "What are you doing here?"

"Reisen, you tell that girl to take me to Kaguya or Eirin before I'll create a portal underneath her feet which will drop her right into the arctic ocean!"

Tewi blinked. "You're bluffing!"

"Yukari never bluffs," Reisen told the smaller rabbit. Turning to Yukari, she said: "Just come along and I'll take you to the mistress."

"Not Kaguya?" Yukari frowned.

"No offense, Yukari… but if the princess is disturbed for no good reason, she can get just as mad and scary as you. Let's just hope Mistress Eirin is in a good mood today…"

The moon rabbit lead Yukari down into Eientei's basement where Eirin had her laboratory. Taking a deep breath, Reisen carefully knocked at the door. "Um… Mistress?"

"Reisen!" came the happy voice of Eirin. "I was hoping you'd come. There's this reeeaaally tricky experiment you could help me with…"

Reisen gulped. "I… I'd love to, mistress, but… see, we have a visitor?"

The door opened and revealed Eirin, wearing a lab coat and a pair of huge goggles over her eyes. "A visitor?"

"Oh no, she's in 'mad scientist' mode again…" Reisen muttered. She then removed the goggles.

The pharmacist blinked. "Oh! It's you…" she frowned when she saw Yukari. "What do you want?"

"I need to talk to your princess," Yukari replied. "As I've said multiple times today. And I don't really have the time for any games, so just take me to her."

Eirin shook her head. "The princess gave me the special order not to disturb her within the next five hours while she's in her room… meditating."

Yukari raised an eyebrow. "Meditating?"

Reisen raised a finger. "Actually, she's playing that new MMORPG she found on the internet and…"

Eiren forcefully put her hand on the moon rabbit's mouth before she could speak on. "Reisen…if you don't want me to test that new medicine on you, you better think twice about continuing that sentence."

Reisen gulped and nodded weakly.

Yukari then walked up in front of Eirin and glared at her. "Listen… I will only say this once: Go in there… and drag that loyal brat off her lazy butt and in front of me, you hear me?"

Both Eirin and Reisen gasped at Yukari's words. "You… you just can't walk in here and make demands…" Eirin stuttered.

Not wasting any word, Yukari created another portal and stepped through… right into the middle of Kaguya's room, who nearly fell off her computer chair in shock.

"Y-yukari!" she gasped. "How d-dare you…"

"Listen up, Kaguya!" Yukari shouted as she stormed up to the bewildered princess and stabbed her with her index finger. "I'm in a very bad mood and I'm here on VERY important business! So you better cut the crap and answer this question: What do you know about the nightmare beast in the moon?"

Kaguyas eyes widened. "The… the nightmare beast…"

"ANSWER ME, KAGUYA!"

Just then, the door opened and Eirin and Reisen came in. "Princess!" Eirin gasped. "I'm so sorry, I will remove the intruder at once..:"

"Eirin, be quiet!" Kaguya ordered in an unusually calm and collected voice. She then looked at Yukari in an inquiring manner. "Why do you need to know about the nightmare beast in the moon, Lady Yakumo?"

Eirin paled. "The… the nightmare beast…"

"What's that?" Reisen asked. No one replied.

"I felt something on the moon this morning," Yukari explained. "Something pressing against one of my borders… a dark force I haven't felt for the past thousand years… a force that the Lunarians were supposed to keep sealed in the moon for all eternity!"

"What are you saying?" Eirin shouted. "Of course we were keeping it sealed! The nightmare beast is an ancient demon, too dangerous to leave it free. As a servant of the royal family, I know that the seal binding it was supposed to stay intact for at least another thousand years."

"Actually, Eirin," Kaguya sighed. "That's not the truth."

Eirin stared at her princess with wide eyes. "What…?"

Kaguya slowly stood up. "No one outside the royal family of Lunaria knows the truth," she said. "We swore to keep it a secret… but then again, I was exiled from the moon… so I don't really have a reason to keep it a secret anymore." She took a deep breath. "The nightmare beast… it escaped. A long time ago. And not only that… it disappeared. We could never find out where it went or what happened to it, but… fact is, the cell we were keeping it in was empty."

"I knew it…" Yukari sighed.

"Princess…" Eirin whispered. "Why didn't you… all those years… why didn't you TELL me?"

"I saw no reason," Kaguya shrugged. "And if the commoners learned that the nightmare beast escaped, it would have caused a panic. It was our biggest fear that it would one day show up to take its revenge… but it never came back."

"It didn't… because it wasn't in this world anymore," Yukari said.

Everyone looked at her. "Beg your pardon?" Kaguya asked.

"I don't know how… but it crossed the borders between Gensokyo and another world the night it escaped…" Yukari's voice sounded very strained. "I knew I felt something… all those years ago… but I never understood what it was… but today, I felt the very same thing… like a knock against the door between worlds… the boundary is in danger…"

"Another world?" Kaguya asked. "What world are you talking about?"

"Gensokyo and the human world… are not the only worlds with a moon, princess," Yukari tried to explain. "When the beast crossed over into that other world… it found itself on another moon. I can't say what exactly happened there… but I felt something this morning. And if my fears are correct… then it wants to return back into this world… maybe it already has!"

"In that case, we need to check!" Kaguya said. All who knew her were surprised to see her for once not acting like a spoiled, lazy brat, but instead like a regent who knew that she was about to face a crisis. "Lady Yakumo… can you bring us to the moon with one of your portals?"

"Princess? You want to go back to the moon?" Eirin gasped. "Now? But the other Lunarians…"

"I couldn't care less about them at this moment, Eirin!" Kaguya said sternly. "The nightmare beast is a horrible creature that cannot be allowed to roam this world freely. Lady Yakumo, know that you have our full support."

Yukari nodded. "I appreciate it, princess… I say we don't waste any time with trying to explain things to your people. I will create a portal that brings us right into the former holding cell of the beast."

"That seems like a wise decision," Kaguya nodded. She turned to face Eirin and Reisen. "Eirin… you stay here and take care of Eientei while we're gone. Don't talk to anyone about this until I'm back… and that goes for you, too Reisen!" She glared at the nervous moon rabbit. "Did anyone else notice Lady Yakumo's visit today?"

"W-well, only Tewi…"

"Lock her up until I'm back. And now, we shall go." She nodded towards Yukari, who created another portal. And then, both of them were gone.

"Uh… what just happened?" Reisen asked. "Was she serious about locking Tewi up, mistress?"

"Kaguya-sama NEVER jokes when she's like this, Reisen…" Eirin muttered. "Let's just hope she and Yukari can do something about this. The nightmare beast… you don't want to know what it's capable of, Reisen."

A dark and empty cave. Nothing out of the ordinary, most people would say. However, this cavern was situated deep below one of the biggest high security building ever built by the Lunarians on the moon, outfitted with the most modern technology you could find in the entire moon capital. Except for the heavily guarded high security shaft that lead here from the surface, dozens of miles deep, there was no entrance and exit. Getting in here without being spotted by any guards or security cameras was virtually impossible.

Still, no one of the Lunarians noticed when a strange gap, filled with a dark red light and a multitude of non-blinking eyes opened in the air inside the cave. And soon after that, two people stepped through the gap…

Yukari took a look around after closing the portal behind her and Kaguya. "So this is the cavern in which the Nightmare Beasts was sealed for eons?"

"Yes, indeed," Kaguya nodded. Yukari noticed that she looked a bit nervous. Then again, it was quite understandable, given the fact that they were directly under the moon capital, the place she had once lived in and was banished from. Who knew what the Lunarians would do once they found out that their exiled princess was walking right underneath their feet?

"Let's finish this quickly," Yukari said as she began to investigate the whole cave. Slowly, she began pacing forth and back through the darkness. For her, there seemed to be nothing out of the usual. "Do you see… or feel… anything that is not like it should be?"

Kaguya shook her head. "No… the security field is intact, none of the walls seem to be damaged in any way. I don't understand how the beast could have escaped this place."

Yukari then stopped in a certain spot. "This is it…" she murmured as she stared down at her feet. "The boundaries… they are very weak at this spot. This is where it must have escaped. Still… the border is intact. Only beings that possess the same powers as me should be able to break them… OR fix them."

"You say this is a border?" Kaguya asked as she approached Yukari. "A border of what exactly?"

"A border between worlds… similar to the Hakurei border. But behind this border lies not the human worlds, but something else… something I haven't seen in a very long time. A world I usually never have contact with…"

"And the Nightmare Beast escaped to that world?" Kaguya asked.

"Apparently… the only way we can be sure is to open a gap and see for ourselves." She held out a palm and invoked her powers… her powers to control all kinds of boundaries. And then, another gap opened up right in front of her.

A gap through which a dark blue mist was seeping.

Yukari gasped as soon as she realized her mistake. 'I'm such a fool…' she thought. 'I thought the Nightmare Beast was lying hidden somewhere in that other world… but instead, it waited just on the other side of the border, on the other world's moon… and lured me right into this trap.'

"Soooo nice to be back in my old world," chuckled an amused voice. "And to see such familiar faces… Princess Kaguya, the exile seems to treat you well."

"It talks?" Kaguya whispered in shock. "It never talked before… it was a mindless beast of destruction…"

"That was before my visit to the world beyond this border… when I took possession of that girl… a foolish princess, not unlike you, Kaguya… I absorbed more and more of her being into myself… her intelligence and personality, merged with my own… turning me into a completely different being than I was before."

Yukari raised her hand and fired a barrage of bright danmaku bullets at the dark cloud. But they harmlessly passed through it. The entity chuckled. "That was foolish, Yukari… I am a being with no body. Only by possessing other creatures can I have my own body. Still, since you seemed so intent on findind me… why don't you become one with me?"

Yukari had only time to gasp when the cloud surrounded her from all sides. And then the impossible happened… the dark smoke began to seep through her skin, into her body. Before she lost consciousness, Yukari could only think: 'I just doomed Gensokyo…'

The dark mist clouded Yukari's body from Kaguya's view, but the moon princess could see that something was moving in there, stretching the boundary Youkai's body to unimaginable limits and distorting her previous figure.

And then the mist cleared… and revealed the new figure standing in front of her. It was an inhuman, four-legged creature with wings as black as the night and a wicked horn on its forehead.

Kaguya stared at the black, winged unicorn in disbelief while the creature chuckled.

"Finally… I am back! Nightmare Moon… is reborn! And now I will take my revenge on BOTH Gensokyo and Equestria! HAHAHAHAHAHA!"

"Not if I can stop you…" Kaguya hissed as she began to float up and focussed her powers. "You made a mistake, demon… you thought you could overwhelm and surprise us with your plan? Too bad for you… I am immortal! You can never kill me!" She chuckled.

Nightmare Moon smirked. "Oh, I don't plan on killing you, dear princess… for now, you can just return to your precious Gensokyo… while I set my own plan in motion! With the new powers of the one I am possessing, I will shatter the boundaries between the worlds! Now BEGONE!" And with a flash of her horn, one of Yukari's portals appeared right behind the surprised princess, swallowing her whole.

Kaguya appeared back in Eientei, landing roughly on the floor of her own room. For a moment, she was dizzy and couldn't see straight.

"Princess?" she heard the worried voice of Eirin. "Thank goodness you've returned… something very strange has happened…"

"Please, Eirin, no matter how strange it is, it can't be worse than what I just witnessed," Kaguya muttered as she tried to get back to her feet. For some reason, her limbs did not quite react the way they were supposed to. "Lady Yakumo… was just possessed by the Nighmare Beast!"

"Well, then this might be just the reaction to that… even though I am not quite certain about the reasons for this kind of effect."

"Just what are you talking about, Eirin?" Kaguya asked as she looked up and for the first time saw Eirin's face clearly.

Staring back at her was a white-furred unicorn pony with same cap Eirin always wears. And when Kaguya looked down at herself… she saw a slightly smaller, pink-furred unicorn with wings.

"She did it… she really did it…" whispered the transformed pony princess.

"Who did what?" Eirin wondered. "You know why we turned into… this?"

"She… Nightmare Moon… she destroyed the boundary between worlds…" Kaguya muttered as she pushed herself up… on all fours. "And now it's affecting our world… she destroyed the PONY BORDER!"