The tale begins several decades following our defeat of the demon serpent known to you as Discord.

One beautiful Summer night, I witnessed the brightest shooting star I had ever seen. It collided with the ground with such force that the Everfree quaked as though in great fear, and a bright discharge of light briefly lit the horizon as though my Sister had just raised the Sun.

The remnants of the star—what one would now refer to as a meteorite—were hastily fetched from the countryside by a contingent of armed guards and alchemists. I prepared my laboratory to receive the artifact. Star Swirl the Bearded was notified, and two of my best research assistants were summoned.

The following day, we had a minor accident.

I stood at the open door of a guest room in the Castle of the Two Sisters, watching a doctor garbed in full antimagic suit tending to a mildly-distressed pegasus at rest in the bed. Due to the capital city of Everfree being located in the middle of an dangerous enchanted forest, quarantine procedures were already well-established at this time. Thus, the doorway to the guest room was blocked by a translucent magical field (my own, as memory serves) which also covered the room's walls, floor, and ceiling.

I heard the unmistakeable hoofsteps of my sister approaching and waited until she stood by my side before speaking.

"We should demure othat the earmfull thearfa beest hale—" ...neigh, one moment.

My apologies, Twilight; I can see by your reaction that your Middle Equestrian is likely rusty, which stands to reason. I was not thinking. Allow me to continue, and I will provide a modern translation.

"I feel I should wait until the victim's health has improved before resuming study on the fallen star," I said to Princess Celestia, as both of us stood watching from the hallway.

"Perhaps that would be prudent," said Celestia. "I have already been informed of his unusual symptoms and the general nature of the incident, but I would prefer to get the details from my sister's mouth. How did this happen, exactly?"

"The stallion you see before you is one of my students: a pegasus student of alchemy, talented in the magic of storms. He was assisting me with the initial inspection of the star when the accident happened. The star has an unusual teardrop shape to it, so we decided to fracture the crust at the inflection point, by which I mean where the spheroid meets the conic segment. You must see the artifact yourself, Sister. It is truly remarkable," I explained, the wonderment not hidden from my voice. "The accident occurred as he guided a ribbon of prime magnus (this means magnesium, Twilight) to burn through the crust of the stellar remnant. The reaction fractured the surface more rapidly than expected, and a glistening black crystal shard from the interior of the star burst out and lanced his leg before I had the time to react and catch it with magic."

My sister gently nodded her head, and stared into space. "This... it is simply extraordinary, Luna. Never before has a stellar remnant of this mass been recovered from Equestrian soil, not to mention a malady sourced from the Deep Sky itself. I have been briefed on your discovery, but I can scarcely believe it. Luna, have you confirmed that actual invisibles (this is how we once referred to agents of infection) are at work here...?"

I nodded in return. "Yes, that which ails him is no mere toxin. There appears to be a form of invisible parasitic life from within the star capable of interacting with pony biology. And, what is more: it is a magical parasite."

"My Heavens, life from within a star! Luna, are you certain we should wait to perform further research? Valuable information may be lost if we allow the sample to grow cold," said Celestia. My sister's enthusiasm would have been contagious, had I not already shared in it. Still, I did my best to maintain a reserved posture.

"Fear not. Star Swirl has placed the broken star under a temporal barrier so that time moves at a crawl for it, but this seems to be an unnecessary precaution we may soon terminate because the invisibles within the star appear to thrive for as long as they remain within its bulk," I said. "Magical scans indicate the invisible forces become inert once they are neither in contact with the bulk of the star's crystalline interior, nor the interior of a pony. It should not be possible to spread them without blood or sexual contact, unless the infection is capable of taking root in plants and beasts. These facts combined suggest that time is on our side."

The pegasus stallion in the bed shifted uncomfortably where he lay, nervously eyeing us as we stood watching. The doctor picked up her bag and walked toward the exit.

"Well, even if more ponies were to become infected from a common plant or animal, there should be no risk of plague since the symptoms appear within an hour," said Celestia. "We must only take care to contain this initial infection."

The doctor exited the field and removed her hood. "A thousand pardons, Princesses," she said. "I hate to interrupt, but the patient is... suffering, so to speak. Might I be permitted to close the door for his privacy, so that he can, um..."

"Fuck himself?" quipped Celestia, with a gentle but mischievous smile. This was well within earshot of the patient, as the magical field did little to muffle sound waves. He blushed, and his bedsheets tented so suddenly that it nearly exposed him. The doctor was surprised by my sister's profanity; I, much less so. Neither of us were embarrassed, however. I quickly shut the door with an shove of magic for the patient's sake.

"Sister, be polite," I chastised. "You need not say such things."

"Ah, but I wish to! It will help ensure his emission be concomitant with thoughts of my backside," she responded, wearing a grin. "Either way, it should do us all some good to lighten up about the situation. Laughter may not truly be the best medicine, but being horny is perfectly normal—even for pegasi. His continued shyness impairs our treatment, as you have just illustrated by hiding the patient from view when this is entirely unnecessary."

"A small number of ponies are bashful about sexual openness," I said, narrowing my eyes. "This is hardly the time to treat him for excessive shyness regarding public masturbation, although I agree therapy should commence at some point in the future. For now, he is a fine student, and has become a victim in service to the Crowns. He deserves the discretion his moral failings require."

"You both have valid points, your Highnesses," said the doctor, her savvy tongue making obvious her long experience in royal service. "However, it is useful that the door is closed, as I have a finding to report which I would rather the patient not hear. There is something happening within his body besides the unusual depth of carnal desire from which he suffers. His tongue, gums, and wings are all radiating excessive quantities of heat, and this appears to be increasing steadily over time."

"Might those not be considered erogenous zones?" I speculated. "That could explain the symptom, at least in small part."

"Neigh, your Highness. From a psychological standpoint, one might consider the tongue and gums to be erogenous, but not relative to the action of the nerves which line them. His entire wing is affected, as well. It is not limited to the erotic bastard wing," she explained.

"I hereby withdraw my good humor," said Celestia, as a look of concern crossed her delicate features. "It is most unfortunate to hear that the infection worsens. Perhaps I spoke too optimistically."

"On the contrary, the infection has all but vanished," said the doctor. "The invisibles are nearly absent from my scans of this hour. His body itself is changing, just not in a way that is yet visually obvious."

"A mutation caused by invisible forces? Fascinating," I said. "But, not a cancer?"

"No signs of that, fortunately; but on the other hoof, at least then we would know what we were dealing with," she responded. "I have no idea what is happening, but it is very possible that potential physical changes to the body are being prepared, in some sense. I consider it likely there will follow a rapid onset of more significant symptoms at some undetermined point in the future. For now, we must continue to observe. I shall order nurses stationed outside the room, and the door shall be opened and remain such once he completes masturbating."

A loud whinnying sound echoed through the door, followed by gasps, then silence.

"I would have this door removed entirely," said Princess Celestia, "as well as the bathroom door within the room. If the appearance of new symptoms is indeed rapid, we cannot allow his foallike embarrassment to jeopardize our observation of the course of a strange new disease." Celestia opened the guest room door just as the pegasus stallion entered the bathroom and closed its door.

"I must agree. I would handle the details," I said.

"Ah, and there is more. Something stranger still," said the doctor, her voice dropping to a whisper. Our eyes fixed upon her. "I tried to let some of his blood for study, and before any could drop, he pulled his leg from the bowl and sucked on the incision."

"He... he what, now? You are telling us that the patient drank his own blood?" I asked.

"Indeed, and it seemed to sate his thirst. His blood tested weak for factors of power. On a hunch, I let some of my own blood and allowed him to drink it—though I did not allow him to drink my humor directly from the wound, naturally. A while later, I let his blood a second time. This time, the strength of his blood had been renewed," said the doctor.

"Ah, excellent! He is a vampony," said Celestia, rolling her eyes. "This solves all of our troubles. To cure him, we need naught but a wooden stake and a few cloves of fresh garlic."

"All legends have a kernel of truth," I said, and both the doctor and my sister (despite the whiteness of her coat) grew pale in the cheek. An awkward silence persisted for a moment.

"Luna, that is remarkably dumb," said Celestia. "This is a disease, not a supernatural farce." I barely withheld my anger, but it likely showed on my face. The doctor held her breath, looked at the floor, and said nothing.

"Do you need to be relieved of your post?" I asked the doctor. "We should get a formal report on these matters as soon as ponily possible, and I would like the nurses you mentioned to be stationed here immediately."

"Two nurses are already due to arrive within the hour. I shall go to prepare my full report once they do," she replied.

"Thank you, doctor. Luna, would you walk with me, please?" said Celestia, and I nodded.

We had walked but a few meters away before I hissed, "Celie! Do not call me stupid in front of other ponies!" at my sister.

"I apologize most deeply, Luna," said Celestia, and her face showed genuine remorse. "I reacted out of shock and fear. I do not like thinking about the possibility that this could be a situation beyond our ability to reckon. The foul serpent claimed he would plant the seeds of disHarmony should we ever defeat him. Perhaps this is his revenge."

I shook my head. "Discord is no longer able to torment us, Sister. He makes for a splendid statue in the Main Hall, though perhaps we should move him somewhere we need not stare upon his hideous face daily," I said. "As for the victim, I am loathe to risk the lives of additional research staff when I can work on this alone."

"You and I should work together," said Celestia as we continued to walk, and her sour expression began to shift back into a smile. "This is something truly amazing, Luna. An invisible, star-born parasite! Who knows what we might discover? Besides, we could potentially find a clue to aid in his recovery. That cannot be delayed."

I sighed. "Well... your point is wise. I suppose you and I would be safe with minimal precautions. It is not like we need split the thing open a second time, and even if we did, we could do so with a barrier in place."

"Exactly. You and I are no foals, especially having been forewarned of the danger," she added.

The conversation would have continued, but suddenly there came a loud moaning and a high-pitched screech, followed by a shriek and a cry for help from the doctor.

We turned tail and galloped at full pace toward our former position. Within seconds we saw it: the first known nightborn in the history of Equestria. It—he, rather—looked much like a nightwing as you know them, but his eyes were red and wild, his tongue long and whiplike as it wrapped itself several times around the doctor's face, and his penis erect and bobbing in midair as it dribbled precum all across the doctor's thrashing hooves where she lay sprawled out across the floor beneath the wind from his flapping, fleshy wings. (It was not until much later I realized how strange the continued existence of the beast's arousal was, given that the stallion had released his emission mere minutes to the previous.)

Princess Celestia quickly restrained the beast with her magic. I, however, stood paralyzed with what could only be described as lust. This was the most beautiful creature I had ever seen, Twilight Sparkle, in spite of the horror—neigh, mayhaps even due to the horror.

The next few minutes were a blur, at least to my mind. My sister made certain the doctor would be placed under observation even though the risk of contamination had been minor, as her earlier wound had been safely bandaged and did not come into contact with the tongue or other fluids, nor did the tongue enter her body. All the loose fluids were carefully contained and burned, despite the fact that they did not scan positive for the invisible factor.

Once I had recovered from my initial shock, Celestia and I decided to place the stallion into magical stasis for his protection. I ordered a bevy of doctors to attend to him, and they began to test experimental treatments.

Celestia and I did not discuss the events until several hours later, while the two of us prepared my laboratory for continued research on the invisibles.

The meteorite was shaped just like a teardrop, with the 'stem' twisted about laterally, rather like a wet towel. When viewed with one eye from the tip down, the cone looked like a perfect disc-shaped spiral (before we fractured it, at least). The far hemisphere of the drop had no striations of any sort apart from its generally mottled, pockmarked surface. From the sightings of the fireball, its landing, and the strength of the impact, there was no doubt as to its Heavenly origin; yet it seemed too perfectly-shaped to be anything but artificial.

In the laboratory, my sister and I spent several minutes planning for an approach to analyze the invisibles within the object's crystalline interior. "I believe we should first attempt to see if the connection between the shard which struck the victim and the crystalline structure has been severed—" I said, not noticing the look of concern on my sister's face until she moved my jaw toward her gaze with a gentle hoof.

"Luna. It is just us, now. You can talk to me about what you are feeling," she said, sitting back onto an uncomfortable stool.

"I do not know what you are talking about," I lied, lowering my eyes to avoid her gaze.

Princess Celestia sighed. "I can see it in your eyes, even now. You loved that creature. This was more than compassion for your servant and friend," she said. "I cannot blame you, Lulu. I was nearly stunned by the view, myself. In a way, it was... beautiful."

I gritted my teeth and stared back up into my sister's eyes, my own blazing with fire. "No, Celie. It was not beautiful in a way, but in EVERY way!" I loudly bleated, my voice aquiver with dissonant energy. "He was glorious. There are no words to describe that mesmerizing, bewitching marriage of Pony and Evening! That stallion is a nocturne within my soul, a beast of fancy with singular claim upon the spirit for which my lungs draw breath!"

Celestia's eyes widened in fear as she placed her shoe upon my lips. "Whoa! Dear Sister, I... I did not quite realize the depth of this when I began my line of inquiry," she said, leaning back in her seat. "Luna... are you thinking of leaving the poor stallion in that state?"

"What?" I said, and paused for a moment (admittedly, in thought). "No, of course not! But... I cannot miss this opportunity, Celie. I must discover a way to unlock the form of the beast without its ghastly vampirism..."

"And without the thoughtless abandon, replaced by beastly lust," said Celestia.

It took another moment for me to reply. "Perhaps a bit of that could yet remain."

Sister covered her mouth with a hoof, and the surprise on her face slowly drifted toward amusement. "Oh... my... goodness. Could this be more than a trivial longing for the aesthetic touch of smooth, leathery wings and fluffy ears? Does my own sister have... a crush?"

"Do not trifle with my emotions, Celie!" I said, barely holding back my rage.

"Ah! But you do!" she said, and I could not tell if the tone in her voice was accusing or celebrating.

I lowered my head. "Fie! So let it be, then. I, Luna, Princess of the Night, wish to mate with a monstrosity of nature from the Stars above," I admitted, then turned my head upwards. I may have been crying at that point, but I cannot recall for certain. "Are you pleased now, Sister? Is my wretched debasement—"

Before I could complete my entreaty, my sweet sister swept me up tightly in her legs and squeezed me with a warmth most welcome to my bosom.

"Oh Luna, I am sorry. I did not mean to poke fun. I am just..." she said to me, and then she stepped back and I could see she was smiling with watery eyes. And what was that unusual emotion laid bare upon her muzzle?

"You... my sister is proud of my unmitigated perversion?" I said, the swelling perplexity within me mercifully blotting out my self-revulsion.

"It is not perversion, sister! You have not loved in decades," said Celestia. "Can you not see? This is the doing of Harmony. This, this thing, it comes from the Heavens above and into your life!"

"N-no, it cannot be," I said, shaking my head. I was afraid to believe it might actually be true, lest my foalish hopes be dashed against the rocks. "The Tree's roots stretch far, but surely not into the Deep Sky!"

"The Tree has done stranger things than this, Lulu. I am certain this is a sign." My sister watched me as I stared off into space, thinking. Nearly a minute passed in silence between us.

"I could try to modify the invisibles," I finally said. "Use them as a catalyst to bring the physical characteristics to bear and make them permanent, while preventing the mental changes..."

"Now that is the passion I have long missed!" said my sister, hugging me about my barrel.

I pushed her off of me. "Sister, no. It would be... perversion to pursue this. Altering the vector might not even cure the disease. I cannot create a new sort of mutation to suit my wicked yearnings," I said. "The Tree would not have me ruin my subjects for a bit of fun."

Princess Celestia sighed at that. "Luna, you are a regent. There is nothing perverse about such a desire, provided you do not violate the will of others. There are many ponies who would thrill to experience such a change," she said. "You will have no end of volunteers."

"You are a foul temptress, Celie," I said, but her words had hit their mark. "You are right in one respect, however. I feel a fire within my barrel which has been absent for many a decade. Perhaps... perhaps once we have a cure for the ailment, I could begin tests with willing volunteers..."

"We do not need a full cure. Just a treatment," said Princess Celestia. "If we can treat the patient with little inconvenience to his life and livelihood, that should be enough."

Pardon as I pause for a time, Twilight Sparkle. I am... simply remembering how much my sister loved me, and how foalish I was to forget that love when I accepted the Nightmare within me.

...

That is better. Let us continue.

Unconsciousness did little to suppress the change, but we soon learned that pressure applied to the bastard wing would cause the lion's share of the vampiric changes to revert and would allow for the subject to regain most of their senses. If fed tiny amounts of blood at regular intervals, the subject could resist the urge to change, but it took great effort. Fortunately, after several days and some assistance from the neighboring zebra tribe, our doctors discovered the effects of aconitum and the vampony's natural resistance to its toxicity. We finally had a treatment.

I begun at once to work on modification of the infectious agent.

It is surprising, in retrospect, that the invisibles turned out to be bacteria rather than virii. We soon learned that the infectious agent could only survive for more than a few seconds in two locations: on the dark crystals of the interior, and within the blood of a sapient host during the initial stages of infection. Princess Cadance is currently researching how the infection spreads through biting, but even a millennium ago we assumed that a small pocket of the bacteria must remain entirely dormant and magically shielded, somewhere within the venom sacs. Those sacs eject a small quantity of blood and (presumably) some of the dormant bacteria, which then become live. Severe damage to a pony's biology may also cause the infection to re-emerge (in particular, removal of the sacs or fangs, both of which will grow back within a fortnight).

With regard to the crystalline material, removing a shard from the interior of the meteorite kills all the bacteria housed on the crystal within seconds. However, touching a broken crystal to any other part of the interior returns the infection to the crystal. This allowed us to infect other ponies for scientific observation without removing plenty of the crystals. To my best recollection, we removed fewer than a dozen crystal fragments throughout the course of all our experiments.

The preliminary findings were nothing short of amazing. We tested the infection on earth ponies, pegasi, and unicorns. Each kind of pony had a unique vampony form (a fact of which you are already aware). We did not test on crystal ponies or people from other nations, due to reasons related to political stability. We did not test on nonpony subjects such as donkeys or sheep due to sensitive racial issues which existed at that time. We did test on plants and semi-sapient animals, but there was no effect. Given the close physiological similarity of certain semi-sapient mammals to ponies, this was very surprising.

Even more surprising was what occurred through one forbidden avenue of my research (Twilight, please do not share the details of this in your Journal). In order to modify the bacteria without affecting the entire sample, I needed them to grow in a host without dying out. I began to experiment with corpses...

I've redacted some of the information from this part of the story for reasons of national security that may persist regardless as to which side of the conflict 'wins' in the end.

You can trust that the data is both nonessential to the story, and useless from a utilitarian perspective (it is impossible to replicate the experiment anyway without access to the meteorite; more on that later).

–Princess Twilight Sparkle

...and thus, the first changelings were born...