In Canterlot Castle...

Princess Luna awoke with a start, her heart racing and her mind panicked. She had been peacefully asleep when she felt something being ripped from her. Not physically, but spiritually. The moon princess stumbled out of bed and flung open her door. She dashed through the sun-filled halls as she galloped towards her sister's throne room.

"Princess, is everything alright?" A guard asked her worriedly as she flew by.

"Indeed not!" Luna replied, not even slowing down. She finally stopped when she reached the tall doors that opened up into the grand throne room. She pushed them open and walked in.

Princess Celestia was shocked by her sister's unorderly appearance, sweating and mane ruffled.

"Luna, is everything okay?" Celestia asked, immediately climbing down and embracing her exhausted sister.

"No sister," Luna panted, "Something is horribly wrong."

"What is it?"

"Nightmare Moon has been detached from me."

***

I woke up, the sun's dying rays falling across my body.

"God, I didn't take a nap again, did I?" I asked myself, startled by the strange voice that exited my throat. Looking around, I wasn't in my room as I expected, but in a dense forest. Remembering everything, I covered my face and growled. Dragging my hooves down my face, I stared at the canopy. Little bits of light shone through, and they burned even though they were weak. Soon the moon would come back out and then I'd be horribly screwed. I had to remind myself I was Nightmare Moon again and that it was natural I slept through an entire day.

"So, where am I anyway?" I asked myself. I knew where I was. I was exactly south of somewhere, north of nowhere, east of everywhere and west of where everyone else is. But what I really needed was a map.

"C'mon Nightie, let's go." I told myself, standing up and choosing a random direction to walk.

"Don't call me Nightie." A voice hissed in my brain. I stood still, trembling like mad.

"W-who are you?" I called out, scared out of my mind.

"Nightmare Moon. I bonded my body with yours."

"Why would anyone do that?"

"I am going to take over the human world since Equestria failed miserably, and humans have weaker minds, so my victory is guaranteed."

"Well, Nightie, you're in for one heck of a ride."

"For the love of Faust, why did I choose this one?"

I just smiled and trotted through the dense undergrowth, knowing I'd have a fun time screwing with her.

***

"How could this have happened?" Celestia rubbed her temple, a cup of now-cold tea in front of her as she discussed the matter with her sister.

"I do not know, but as I recall, Nightmare Moon always sought independence." Luna explained, taking a ginger sip of her third cup. "However, she does not truly understand what it means to be independent, thus forcefully thrusting herself into the body of one who seeks the same."

"I thought Nightmare Moon was a part of you." The elder spoke, quite puzzled.

"Indeed, she is, was, but she merely usurped my body." The small blue alicorn answered, a dark look crossing her face as she remembered what happened that thousand-some years ago. "She remained in the darkest depths of my mind, hence why I could turn into her for Nightmare Night."

"Can you track her?"

"Possibly, but she is not like us. We are merely powerful, she is a goddess, and a corrupt one at that. She could reject both of our attempts to track her. She could have also gone to a different world, one where her magical signature would be untraceable."

"But we must try." Celestia urged her sister, nearly knocking over her tea as she slammed her hooves on the table.

"Indeed we must." Luna smiled weakly.

***

"Nightie."

...

"Nightie."

...

"Night-"

"WHAT?!"

"Can you read my mind?"

"Yes,"

"Is it disturbing?"

"Immensely."

So the immortal horse goddess of the moon who wishes to plunge every world into eternal darkness has took over my body and wishes to use it against earth. What do I do? Get the mental image of her in a bunny suit to mess with her.

"Stop."

Now she's singing the ABC's to watering cans while hopping over them.

"Stop."

"No."

"I AM NIGHTMARE MOON AND I COMMAND YOU TO STOP!"

...

"No."

"You insolent foal, do you know what I can do to you?"

"You can't even control the body, I am." Suddenly, my legs seized up. I started doing the chicken dance while cawing.

"Do we have a problem?"

"N-no!" I smirked, or rather she made me. I quickly moved through the forest, keeping silent and my mind clear. Pain erupted from my legs as I didn't have the armor, and my hooves were soft from never being used before.

"You are a wimp."

"What did you call me?" I asked, my voice dangerously low.

"A wimp, is that not what you are?"

I got a running start at a tree, using my wings to gallop up the trunk. I flipped upside down and plunged my horn right in the middle of a hoofprint I left on it.

"Oh. So you're not a wimp, you're an idiot."

"Eeyup!" I smiled, proud of myself. I tried to pull my horn out, but it was stuck.

"Are all humans like this?"

"Some are more stupid." I felt my hoof hit my face so hard I blacked out.