Present day, Neighagra Falls, Equestria...

Princess Luna took a sip of cider and looked around the White Dragon Tavern. The wood-paneled walls were lined with antiques - toys, household items, fishing rods, and a large propeller from an unknown aircraft. A trio of Earth pony farmers discussed crop prices in a corner booth. The only other patrons were a pair of scraggly-bearded Earth ponies who were playing cards. The tavern keeper, a platinum blonde Earth pony mare with a mug of cider as her cutie mark, smiled at Luna, who began to giggle as she saw her own reflection in the mirror behind the bar.

Luna's sister, Princess Celestia, had asked Luna and the unicorn wizard Star Swirl the Bearded to help research the dark entity that had recently corrupted the Unliving Queen, an undead Twilight Sparkle imbued with unholy magic from an alternate timeline. The Queen and Star Swirl had traveled back in time to prevent a terrible cataclysm that Star Swirl had foreseen in the future. During this quest, Twilight had succumbed to a corruptive influence, ultimately resulting in her undeath, The corrupted Twilight had fled back into her own timeline, where she ruled as the Unliving Queen of Equestria. The Queen had been defeated days prior thanks to the power of the Elements of Harmony and the magic of friendship.

Star Swirl had suggested they travel incognito, and Luna decided to make it fun by having them choose the other's illusion. For Luna, Star Swirl had selected a svelte pegasus with an electric blue coat, an amber mane streaked with black, and a cutie mark of a storm cloud spewing lightning. It was in stark contrast to her normal, dark appearance, and she had to get used to not using magic to manipulate objects, but Luna was having fun with it.

Luna had been excited when Star Swirl had proposed their first stop to be Neighagra Falls. To Luna's disappointment, Star Swirl had selected the location not to enjoy the majestic beauty of the waterfalls, but to delve into the archives at Neighagra University. However, Luna was a princess, thus she could issue orders as well, and so she had commanded the old wizard to abandon the dusty tomes and meet her at the tavern to enjoy a meal.

A bell jingled as the door to the tavern opened and a unicorn stallion entered. He had a burgundy coat and a sepia mane which poked out from beneath a ten-gallon hat. A lariat was his cutie mark, and he also sported a well-groomed handlebar moustache. His horn began to glow as he pulled off a pair of dark sunglasses and slipped them into the pocket of his duster. He glanced around the tavern, then walked over to Luna's table, his spurs jangling.

Luna placed her hoof over her mouth to try and suppress her laughter. "Sorry, 'Big Hoss.' I still get a case of the giggles when I see thi-" Luna had almost slipped and used the archaic 'thine,' something which would have surely helped blow her disguise had anypony been paying attention to the pair, "your illusion."

Star Swirl grinned and glanced down at his garments. "At least you seem to be enjoying it..."

Star Swirl felt his stomach growling and rubbed his hooves together, "I didn't have a chance to get any lunch today I'm afraid. My humble thanks for inviting me to supper, my friend."

The tavern keeper arrived with a mug of cider for Star Swirl, then nodded at Luna. "I'll have your meals out shortly, sugar."

Luna smiled, "Thank you." As the tavern keeper headed back to the kitchen, Luna turned to Star Swirl. "I hope you don't mind, I took the liberty of ordering for us both."

"I have faith in your judgment," Star Swirl said as he magically lifted his mug. "Practically anything would be most excellent right about now."

"So, what is so important that you've decided to spend your vacation pouring over tomes instead of spending time with me?" Luna asked.

Star Swirl took a long swig of cider then wiped at his mouth. "I can not get Nightmare Moon from my thoughts."

Luna raised her eyebrow. "Should I be flattered?"

Over a thousand years ago, Princess Luna had become a wicked mare of darkness known as Nightmare Moon. The official story was that Luna, who was responsible for raising the moon and bringing forth the night, had been filled with resentment and envy towards her big sister because the ponies relished and played in Celestia's day, but shunned and slept through the night. Because of this, Luna had become Nightmare Moon and sought to bring about eternal darkness and used her dream walking abilities to plague the ponies with horrendous nightmares. Her own sister had to use the Elements of Harmony against her, imprisoning her on the moon for one thousand years. After her prophecized escape from the moon with the help of some stars, Nightmare Moon had briefly terrorized Equestria once more until Luna had been cleansed by the Elements of Harmony, used by Twilight Sparkle and her friends, who were the new destined bearers of the powerful Elements.

While Luna had been hurt by the unintentional actions of the ponies of yore, it had been long suspected that some unknown entity helped corrupt her and ushered in her transformation to Nightmare Moon. Princess Celestia had been unable to track down the responsible party, and so she kept that detail secret, lest the populace become frightened over the prospect of an unknown, evil entity able to corrupt even a powerful pony such as Luna.

"Perhaps a little of both, I'm afraid," Star Swirl said. "I've been thinking of her and the Unliving Queen. There was something about the Queen that seemed familiar, but I couldn't place my hoof on it. Since I've been thinking about it, I've come to the conclusion that Nightmare Moon and the Queen have something in common. I believe that whomever, or what ever, helped turn you into Nightmare Moon was responsible for the Queen as well."

Luna slowly nodded. "Since my return, Celestia has always told me that it wasn't my fault..."

"What can you remember of your time as Nightmare Moon?" Star Swirl asked.

"My memories of my descent into Nightmare Moon have always been hazy," Luna said. "My emotions I can still recall: Sorrow, anguish, fury. When I became her, though, it was the most peculiar feeling. It was as if I was in a dream, watching myself say and do things that I never could have - never would have - done on my own. It was empowering, I hate to admit."

The tavern keeper arrived bearing several platters. Luna had ordered grilled Portobello mushrooms stuffed with herbs, roasted potatoes and onions drizzled in olive oil and dashed with garlic, and fresh asparagus.

Star Swirl eagerly dug in to his meal before responding to Luna. "As if in a dream.. Yes, that would seem to make sense. I believe you and my wicked little Twilight Sparkle were victims of some kind of insidious force. While trying to prevent the apocalypse I had foreseen, I had spent a good amount of time with Nightmare Moon in various situations."

Luna narrowed her eyes, "Oh, really?"

Star Swirl nodded. "Indeed. I don't know what happened to our timeline, but it was a mess. Mi Amore Cadenza had used her marriage to Shining Armor to steal the throne from you and your sister, citing an ancient law about royal marriages. It was horrible, she turned the populace against her 'enemies' such as Twilight and her friends... I felt if I could prevent you from becoming Nightmare Moon in the past, perhaps that situation would not have come to be. During these efforts, I learned that regardless of any factors that may have influenced the Luna she was born from, Nightmare Moon was always the same. Identical personality, identical behaviorisms. Quite frankly, I believe Nightmare Moon is in part her own being. Why this being attacked you, or where it came from, I do not know. I believe it is most definitely intelligent, unfortunately, as it always evaded detection, no matter how thorough my observations were."

Luna frowned. "And this same thing, this being, did that to your Twilight Sparkle and helped twist her into the depraved Unliving Queen?"

Star Swirl nodded. "I am not positive, but it is likely. When you became Nightmare Moon, your personality and vernacular changed. You certainly didn't seem like yourself. So too was the Unliving Queen rather un-Twilight. She had even gone so far as to believe herself a completely different pony from Twilight Sparkle."

Star Swirl leaned close to Luna. "And it might be that she was in fact not Twilight Sparkle. At least, not entirely."

Luna gnawed at her portabello. "What do you mean?"

Star Swirl pulled a tome from his coat and set it in front of Luna. Luna could tell the tome was quite old and she frowned as she read the title: Demonology. "Demons? Those don't really exist, do they? Ponies used to shun my night in part because they believed demons lurked about. I never found any, despite my efforts."

"They were very real, I'm afraid. Our ancestors encountered them long before you, or even myself, were born," Star Swirl replied. "I've been reading all about the crusades our tribes launched against them. They ultimately forced all the demons they could find back through the portals to the Netherworld - the demons' magical homeland that parallels our world - and then sealed them up. Evidently they persisted as folklore, as you can account, and perhaps some persist yet, and are more than just stories used to frighten foals."

Star Swirl used his magic to flip open the tome to a certain page. "Take a look at this."

Luna began to read the text. "Demonic Possession. Demons are capable of controlling a pony by possessing their body. The victims often suffer erased or altered memories, altered personalities, access to hidden knowledge, the ability to predict the future, the ability to speak languages unknown to the victim, the sudden appearance of wounds and injuries upon their body, and drastic changes in their vocal intonation and behavior. The victim often gains supernatural strength or a boost to their own powers, however the demon usually just inflicts their host with a number of maladies, including diseases, blindness, and loss of speech. The victim has no control over their demonic guest and so it will persist until the demon chooses to or is forced to leave the victim."

Luna glanced up at Star Swirl. "That does sound similar to what I endured as Nightmare Moon, and poor Twilight Sparkle..."

Star Swirl nodded. "I wished to come to Neighagra as the University has quite the demonology collection, one of the few sources of demonic knowledge in all of Equestria. It was just a hunch, but a demon attack seems quite plausible, does it not?"

Luna sighed then finished her meal, pushing the plates to the side. "If there is a demon out there attacking ponies, then it is my duty to stop it. I can not say if your hunch is correct, but I also can not risk letting this thing run rampant and harm anypony else. We must return to Canterlot and inform my sister at once."

Star Swirl reached across the table and put his hoof on Luna's shoulder. "My apologizes, your highness. I know you wanted to see the waterfalls."

Luna gently patted his hoof, "It is okay, Star Swirl. While it can be nice to get away from it all for a time, I know I have responsibilities I must adhere to. Especially since my poor sister had to perform my tasks in addition to her own burdens for a thousand years. I am indebted to her, even if it was a demon that caused my fall from grace, and if I can relieve some of Celestia's stress, then I must do so, for she deserves that in spades, as they say. ...Do they say that?"

"Indeed," Star Swirl said. He leaned back and sipped at his remaining cider. "You and Twilight Sparkle were likely very enticing targets for this demon, and...." His face paled as he sat up. "By the stars...."

"What is it, Star Swirl?" Luna worriedly asked.

"If Nightmare Moon was possessed by a demon, that would mean that I, and more importantly, the Star Swirl from your timeline, may have succumbed to demonic influence due to our interactions with her. An influence that would perhaps lead him to poison your niece," Star Swirl said.

The apocalyptic future that Star Swirl had endeavored to prevent had begun when Luna and Celestia's niece, Mi Amore Cadenza, had used a loophole involving marriage to claim the throne of Equestria. The Star Swirl of the past had been trying to prevent the future disaster as well by attempting to get Luna married before she became Nightmare Moon, and had resorted to a love potion which turned out to be a love poison that Mi Amore Cadenza drank instead. Because Mi Amore Cadenza's special talent was love magic, the poison had an adverse effect, turning her into a raging mare of envy. Star Swirl and the future Twilight had traveled back in time to warn everypony, and Twilight Sparkle and her friends cleansed Cadenza with the Elements of Harmony.

It was this incident that ultimately led the future Twilight down the path that ended with her becoming the Unliving Queen, as well, for she and Star Swirl hailed from the future where Cadenza was not cleansed, resulting in much heartbreak for Twilight as the world she knew crumbled around her as she desperately tried to save it. In an attempt to prevent her past self from having to experience the same things, the future Twilight had set off on the path to become the Unliving Queen in an attempt to create a perfect future for herself.

The pair fell silent as they both mulled over the implications. The tavern keeper cleared their empty plates and brought them their dessert - fresh peaches in a cream-and-brandy sauce.

"If something did influence the other Star Swirl, and possibly you as well, it would seem you both fell right into its trap," Luna said solemnly.

"Part of me hopes we are wrong about all of this, as I don't much like being played for a foal," Star Swirl said as he idly ran his spoon around his dessert dish.

As Luna savored her dessert, she grinned slyly at Star Swirl. "We shall depart for Canterlot immediately, after finishing this scrumptious dessert, of course."

Star Swirl smiled back at her between bites. "Of course, your highness."