When Star Swirl the Bearded and Princess Luna arrived back in Canterlot they had immediately sought out Princess Celestia. They had found her in the Canterlot Archives, reading through a stack of tomes. The elder of the two Equestrian princesses, Celestia had a long, beautiful mane and tail of hues of blues, pink, and turquoise that flowed on their own. Her cutie mark was of the sun itself and looked splendid upon her coat of white. Celestia saw the illusions her sister and friend were disguised in and started to giggle. "My, my, just what were you two up to?"

Luna rolled her eyes and dispelled her illusion, revealing her true self. In contrast to her sister, Luna had a dark blue coat and a flowing mane and tail of lovely blue that twinkled with stars like the night sky. Her cutie mark was of the crescent moon upon a dark cloud-like shape. Star Swirl likewise dispelled his own illusion and the cowpony façade faded, revealing a unicorn with a charcoal coat. His white hair was unkempt and he had a long beard. His cutie mark was of several stars around a swirly design.

Star Swirl's horn began to glow and a robe and hat appeared on his person. Both garments were patterned in shades of blue, adorned with star and moon symbols, and were trimmed with bells. Star Swirl bowed. "Princess Celestia, we believe we may have discovered something about what really happened to your sister one-thousand years ago, and now more recently to my Twilight Sparkle."

Celestia nodded, knowingly. "I thought you might. Luna does not like to think about Nightmare Moon, and I do not press her on the matter. However, given what occurred, I figured you would not rest until you figured it out, and by putting you in close proximity to Luna your mind would inevitably dwell upon the similarities between what befell your Twilight Sparkle and Nightmare Moon."

Luna laughed. "Aha, I have fallen afoul of one of Celestia's famed machinations, yet again!"

Star Swirl conjured forth the Demonology tome and set it in front of Celestia, opening it to the page about demonic possession. "We believe the thing that attacked Luna and Twilight Sparkle may be a demon, your highness."

Celestia frowned as she read the text about demonic possession. "Hmm. I didn't think those actually existed beyond fairytales, but I suppose I should have known better. See, that is why I sent you two out there. I knew the two of you would come up with something. I've been scouring the Archives, but had no luck on my own."

Luna examined the tomes her sister was reading - a variety of ancient bestiaries. Celestia began to use her magic to place them back on the shelves. "I've read about all manner of beasts most foul, but nothing sounded like what we are dealing with."

Celestia used her magic to hold up a piece of parchment. "I've begun writing my own entry on this entity. Insidiosus Diabolicus, a corruptive force of unknown origin. It seems to target ponies that are emotionally weakened, where it then twists and corrupts them into an entity of its own design, yet the victim retains most of who they originally are."

Star Swirl stroked his beard. "I'm ashamed to admit the idea never crossed my mind before, despite my dealings with Nightmare Moon. What I find interesting is that you apparently knew, not entirely, but you knew something besides your sister's jealousy was to blame."

Celestia laughed. "I know my own sister. Nightmare Moon may have shared her body, but that wasn't Luna, not entirely. Unless you believe my sister would go from saying thou's and thee's to you's just because she went dark side on us."

Luna smiled, then began writing on a piece of parchment. "We know this thing may have possessed me one-thousand years ago, up until a few months ago when Twilight and the girls used the Elements against Nightmare Moon and cleansed me of my dark powers. In Star Swirl's timeline, the entity somehow wound up within Twilight Sparkle. Her journal suggests a more recent infection, but what if it wasn't? What if on the night of my return from my lunar prison, it moved immediately from me to a new host - Twilight Sparkle?"

Celestia frowned. "One of the telltale signs of possession is an altered personality, but our Twilight has been the same old pony I love as far as I know."

Star Swirl nodded. "The tome has some methods to check for victims of possession. We can check her body for any mysterious wounds. Try to smell sulfur around her. Question her on events in her life, see if her memories were changed."

"I don't want her to be worried, though," Celestia said. "She's had enough doom and gloom. We need a non-intrusive way of checking her."

Luna grinned. "Why, it's simple! The Elements! Remember, sister? The Elements denied me as their bearer once I started to become Nightmare Moon. We just need to bring the Element of Magic to Twilight and see if it returns to its dormant state."

Celestia gently rubbed her horn against her sister's. "Good thinking, Luna. I'll go get the Element of Magic, you two scry for Twilight."

As Celestia left to the Vault of the Elements, Luna and Star Swirl conjured up a map of Equestria and began channeling a scrying spell to track down Twilight Sparkle. They found Twilight at what appeared to be the home of her brother, Shining Armor. As soon as Celestia returned, the trio set off to Shining Armor's home, which was located in one of Canterlot's affluent neighborhoods.

A pair of sentries were stationed outside Shining Armor's manor and they snapped to attention and saluted as the princesses arrived. Celestia nodded at them and then knocked on the door.

Twilight grinned as she opened the door for them.

"Princess Celestia! Princess Luna! Star Swirl the Bearded! To what do we owe this pleasure?" Twilight joyfully exclaimed.

The three ponies joined Twilight in Shining Armor's foyer and Celestia produced the Element of Magic. The Elements of Harmony were the most powerful form of magic known to ponydom, and Twilight represented the most powerful of the Elements: Magic. When Twilight and her friends had become the new bearers of the Elements, the Element of Magic had assumed the form of a tiara. Celestia set the Element of Magic on Twilight's head and the three stared anxiously at it.

"What is going on?" Twilight asked.

"Cast a spell, please," Celestia requested.

"Um, okay..." Twilight said as she began casting a simple spell. The three peered intently at her magical field.

Luna shook her head. "I sense nothing unusual. Sister?"

Twilight tried to back away as Celestia mischievously licked her horn to check for corrupted magic, which had a tell-tale sulfur taste. "Thankfully, I can't detect any aberrations."

Twilight teleported herself across the room and began wiping her horn with a cloth. "Blah! Uh, no offense, but you are kind of creeping me out. What is going on?"

Celestia giggled. "Sorry, Twilight. We were afraid you might have been possessed by something evil. The same thing that created Nightmare Moon and, most likely, the Unliving Queen. We don't know what it is, but it is a force of corruption. It appears you are safe, thankfully."

Twilight frowned. "You say something did that to Princess Luna and the future Twilight? Why would something want to do that?"

Star Swirl shook his head. "We don't know, but we are going to find out. Rest assured, Twilight Sparkle, whatever this evil is, we shall have justice upon it. Our apologies for startling you, Twilight. We shall let you get back to your brother."

Twilight shook her head. "No, if something is out there, something evil, something that might be trying to corrupt me, or Cadence, or worse? I want to help find it."

Celestia smiled at Twilight and softly nuzzled her for a moment. "Very well, my faithful student. Let us return to the Archives and see what we can learn."

***

Back in the Canterlot Archives, Twilight eagerly began pouring over the Demonology tome as the princesses and Star Swirl discussed the Insidiousus Diabolicus.

"Okay," Luna said, flustered. "If it isn't within Twilight Sparkle, it could be anywhere!"

"Knowing where our enemy is not, is as vital as knowing where it is," Celestia said. "I know I feel a little more at ease knowing this thing didn't go after my little Twilight."

"We could use time traveling spells to try and figure out where it is, and what it is," Star Swirl offered. "I didn't have much luck before, but I'd be willing to try again."

Celestia shook her head. "It's too dangerous. For one, it may target us instead. Secondly, if we did somehow alter the timeline, think of what disastrous consequences there could be. If my sister didn't become Nightmare Moon and end up banished for one-thousand years, she wouldn't have become unsealed at just the right moment for Twilight and her friends to form their bonds. Who knows what we might encounter coming back to the future. We could be without bearers of the Elements, or worse."

Star Swirl sunk deep in thought. "Everything does seem to keep coming back to Twilight Sparkle, doesn't it?"

Celestia put her hoof on Twilight's shoulder as her student smiled up at her. "Well, she is a very special little unicorn."

"I have an idea," Star Swirl said. "You were looking in bestiaries, but that's not what we need. We need to look in history books, town records, newspaper archives, things of that nature. See if there have been any other ponies that suddenly became corrupted like our dear Luna here."

Over the next hour, the four poured through all the historical documentations they could get their hooves on. They began compiling a list of probable cases where the Insidiosus Diabolicus was involved. Unicorns, Earth ponies, and pegasi had all potentially been targeted. All of them had been experiencing turmoil in the minds according to witnesses that knew them, before suddenly becoming a shadow of their former selves and causing trouble. Most had to be zapped by the Elements of Harmony before they returned to normal as if nothing had happened. The incidents had been occurring infrequently for thousands of years.

Star Swirl sat back, rubbed at his weary eyes, and conjured up a glass of water. "I don't understand what this thing is. These attacks it may have made, there's really no rhyme nor reason for some of them. It wanted eternal darkness with Nightmare Moon, a world of unholy magic or some such with the Unliving Queen, yet here the possibly corrupted host was just trying to abolish apple cider. Or here, this crazed Earth pony traveled Equestria and forced ponies to plant eggplants."

Celestia looked at her sister. "Perhaps the entity itself has no goals, other than corrupting ponies. The desires of the corrupted victims may be the desires of the pony it attacks. For instance, Luna may have wanted eternal darkness so the ponies would be forced to appreciate her night. Or Twilight Sparkle may have sought undeath due to the orderly nature of death and the power it would bring so she could save her past self."

"Since she ended up controlling the undead and used them to enforce her rules upon everypony under her, I think it was the 'orderly' part she would have most wanted," Twilight offered. "I like everything being arranged how I like it to be, whether it's the books in the library back home at Ponyville or my beauty products in my room. I'm... kind of obsessive about things. Sometimes if somepony moves something of mine I just go nuts until I move it back."

Star Swirl nodded. "Well, regardless of its motives, it would appear to be escalating its attacks. Hopefully next time it strikes we will be able to retaliate."

Luna shuffled the papers in front of her into a stack. "We now know more about it. It can not hide from us. We will find it and find a way to destroy it! Now, if thou shalt please excuse me, I have an appearance I must prepare for tonight."