Dear Twilight Sparkle,

I regret to inform you that what you ask for is impossible. Such spells no longer exist, as that they have been systematically removed from the repertoires of pony knowledge.

As you know, historically fillies were preferred to colts due to laws requiring a female heir and notions, completely unfounded and fallacious notions, that mares were inherently more intelligent and and magically capable than stallions.

However as you might not know, at certain times it was common practice by much of ponykind for spells to be used to permanently transform newborn colts into fillies using magic. Unfortunately this practice had serious repercussions; the magic lingered in those changed causing them to birth fewer colts. And as such magic does not dilute with the generations, the imbalance only grew as more colts were transformed.

Due to this, all spells meant to change a pony's sex from male to female were removed from all spell books and made illegal. But even with the stopping of the practice the accumulation of the magic still caused the near extinction of ponykind. In fact, the results are still felt today where the ratio of mares to stallions varies from as low as four to one (relatively speaking as four to one is still wildly unbalanced compared to the average of one to one that has been true for most of pony history) in places like Manehattan to as high as twelve to one in places like Ponyville.

This brings me to my next point: while I am sure that it would be well within your ability to recreate the aforementioned spells, the use of any spell that permanently transforms a stallion into a mare is highly illegal. The fact that you were originally a mare would not be accepted as a defence. And as the transformation would be considered by the courts of pony law to be high treason (and nearly a dozen different other crimes) there is nothing I could do to protect you, never-mind the disaster you could potentially cause by reinventing such a spell.

Unfortunately I cannot answer your other question either, as I have no idea why that spell would have had the effect of permanently changing your sex, then again Starswirl the Bearded was a True Genius, unlike anypony either Luna or I have ever known, and was always incredibly poor at describing how the spells he created worked. Nor was he the kind to do anything the easy way, in fact he seemed to take a perverse pleasure in making his spells as complicated as he possibly could. I often thought that he would be happy if nopony was ever able to use any of his spells.

On a much lighter note, you should spend some time getting used to your new body; you might just like it.

Your Loving Mentor,

Princess Celestia

Twilight reread the letter hoping that it had been misread the first five times and let out a sigh, a deep baritone sigh rather than the alto that was familiar. This was not the first time that she, or rather he, had been told he couldn't do something; however this was the first time he had been told that it was illegal rather than impossible.

He had done many impossible things: hatched a dragon egg, redeemed Princess Luna from the Nightmare, 'vanquished' an ursa minor and even become an alicorn. But this time it was far from impossible – Princess Celestia had said that its use had been rather widespread – however this time it not impossibility but rather legality separating him from his goal.

High treason was one of the only crimes in Equestria that still could carry capital punishment. He couldn't imagine somepony being hung for changing their gender, yet there it was. Granted the punishment seemed slightly less absurd when viewed against the problems that gender-flipping spells created (which was most likely why Celestia had chosen to include her little history lesson). But still, High Treason!

Being a princess (or rather, a prince) came with a certain immunity when it came to criminal, civil and magical law (not that Twilight ever had the need to use it ... much) but not even Celestia herself would be able to get away with high treason.

It was truly infuriating that he –alicorn prince of Equestria, Element of Magic, Duke of Ponyville, high lord of the noble House Twilight, arch-Magus on the unicorn Council of Magic, one of the most powerful magicians in history, the premier mind on no less than seven scientific and magical fields (three of which were pioneered by the lavender unicorn-turned-alicorn mare-turned-stallion) and many time hero of Equestria – could not use a simple transfiguration spell (in this case simple is extremely relative considering even the most simplistic transfiguration spell was several levels of difficulty beyond what most unicorns could do) because it's bucking high treason.

Now many spells are what most ponies consider illegal – like raising the dead or capturing a pony's soul and forcefully removing it from their body – but the spells were not illegal; rather the effects were (for example the soul manipulation spells can be used in many perfectly legal ways such as the healing of a fractured soul). In addition the spells were not criminally illegal (unless they were used to commit a crime) but rather forbidden by magical law and was therefor the jurisdiction of the Council of Magic and not the guard (a fact that was well known in House Twilight considering Twilight's (Or Sparkle as called by family members to avoid the inevitable confusion as to precisely which Twilight they are talking about) position as an arch-Magus and Shining Armour being the captain of the Royal Guard). Thirdly laws surrounding forbidden spells were not in any way set in stone and Twilight's position as prince and arch-Magus (not to mention former Element of Magic) would be enough to get away with the usage of all but the most heinous spells (a good example would be the corruption magic that she had used when searching for the crystal heart, and that was before becoming a princess).

But this spell was criminally illegal! If he went through with it's creation and use the nobility would be baying for blood. He (or she, depending how far Twilight got before getting caught) could be imprisoned, or banished, or imprisoned in the pace he was banished to (and for the first time in Twilight's life is this statement not a neurotic irrational exaggeration) or even executed (this is highly unlikely considering Princess Celestia's personal view on capital punishment and her relationship with the younger prince (no, not in that way) but logic is something that seems to loose itself in the mind of a panicking Twilight Sparkle)

His life would be forfeit. He would be hung, no drawn and quartered, no burnt at the stake, no tarred and feathered, no ... (Twilight had unfortunately recently read a book about the many highly 'imaginative' ways that ponies had used through history to dispatch their unwanted that went into extremely gruesome detail so therefore we will pretend the next few thoughts that want through his mind never happened) ... and the execution would take place in the Canterlot square with the entirety of Equestria there to witness his shame as he becomes the first pony to be executed in over 400 years. Of course many, if not most, of the the citizens would wonder what their prince had done to deserve such a sentence. For many the sentence would describe some utterly terrible crime, while those who know the truth would see a martyr for whatever cause they so choose.

The panic started to recede after the scenario had fully played out. Twilight would never go ahead with the spell when such heavy consequences could lay on such simple actions. However as the panic fled, it's place was taken by a new emotion, anger. She would be living her whole life in an unfamiliar body; she would be living as a he. All because of one bucking law. And it is not like Celestia couldn’t just remove that little law with one small decree (in truth it would require more than that, but it would still be fairly easy for the one hailed a the 'God Empress of All Ponykind'). Why wouldn't Celestia help him? And then there was the last line of the letter “you should spend some time getting used to your new body; you might just like it” it was almost like Celestia intended this to happen. And it was infuriating. Here Twilight was in unfamiliar territory and Celestia comes and bars the way back saying “you just might like it”. It was almost as if Celestia had banished Twilight from maredom and “you just might like it” means “sucks to be you; you will will never get back in, so get used to it”.

How dare she? Celestia had no right to do this to him; he wasn't technically even one of her subjects. Of course this could be a test. Another bucking test. Twilight was tired of Celestia's tests. In her mentorship under Princess Celestia there was always a test hidden around every corner and Twilight had often been blindsided by one. It was extremely stressful to never know when the next one was.

The tests continued even after she had moved to Ponyville; except then it was not only stressful for Twilight but for her friends too (a good example would be the entire fiasco with the Grand Galloping Gala (now one of Twilight's least favourite alliterations) starting with Celestia giving her two tickets when she had five friends and ending with Celestia telling them that the only reason that they were invited in the first place was to ruin the event for Celestia's amusement). However, Twilight just continued to be the faithful student blindly accepting whatever was thrown at her and her friends (be it “go talk that big fire-breathing potentially-carnivorous dragon into leaving his cave” or “I forgot to mention that the bird was in fact a bucking phoenix and you neither need to nor can help it Fluttershy”)

However the events that caused Twilight to finally wake up was her brother's wedding and the return of the Crystal Empire. When Twilight came out and confronted 'Cadence' (rather hysterically, but still confronted), and her friends abandoned her (she had specifically remembered a letter to Celestia about taking your friends problems seriously), her brother disowned her (he was under mind control so it wasn't his fault) and she was at her weakest point in quite possibly her whole life, Celestia didn't try to comfort or even talk to her. She put on the same face she does when confronting murderers and rapists (both are, thankfully, very rare in Equestria, however every one is taken directly to Celestia for trial) and said only “Twilight I am disappointed in you”. While her friends apologized to her as soon as the invasion was over, Princess Celestia couldn't even comprehend that she had done something wrong (to make it worse the Princess knew it was changelings that had threatened Equestria but failed to mention it to her student because she 'didn't want her to worry' and was so sure that nothing could get in that she didn't even conceive the idea that somepony might be an imposter).

Then three weeks later Twilight was called to Canterlot. Twilight thought Celestia had finally ponied up to apologizing but she had quickly learned that this was to be another test. Twilight and her friends were going to go into unknown territory to defend an unknown city in an unknown way from an unknown hostility with unknown abilities and the Elements of Harmony were to be kept safe in their vault in Canterlot (because it's not like they cannot be used without their bearers; oh wait, they can't). To make it worse, not only were they completely ignorant and unequipped, the entirety of Equestria hung in the balance. And that was her test.

So it was reasonable to believe that this was yet another test (especially since Celestia had been the one who sent her the spell in the first place), and Twilight was furious; throughout the previous tests Twilight had always been herself, but now the test had changed Twilight and changed him permanently. This was the last straw. Twilight had dreams of eventually finding a nice stallion and starting a family, being a mother. Now that dream had been ripped away. Twilight started to feel the anger build up to a boiling point. In a deep part of his mind he knew such powerful anger was dangerous, but he ignored it. He deserved this anger. It was righteous anger. His life had been unceremoniously bucked up by one spell and one law. The worst part was that if Celestia had really wanted to help she could have had that one law removed near instantaneously yet the eldest princess just sat on her flank and did nothing. How dare she? How! Dare! She!

Twilight was shocked out of his thoughts by the spontaneous combustion of the Princess's letter. He watched as the scroll was consumed by brilliant violet flames and left a small pile of ash on the floor.

“Soooooooooo, bad news?” Twilight had almost forgotten that Spike was in the room

“What tells you that?” Twilight responded with his trademark snark. The (admittedly melodic) baritone of his voice grating at his ears.

“So, the Princess didn't have a spell”

“No”

“So, you are going to spend every hour of the next few days manufacturing a counter-spell?”

“No.” The defeat was evident in Twilight's voice

“Why not?”

“Because that would be high treason”

Spike instantly fell to the floor laughing. “Oh, Twilight you had me going there.”

Twilight stared straight into Spike's eyes and the young dragon ceased his guffaws “I’m not joking.”

“But. But, high treason, that's absurd”

“Yes. It is completely absurd. Completely bucking absurd!”

“What are you going to do?”

“I, I, I don't know” At this Twilight broke into quiet sobs. He looked around the room. It was a large stone room with almost every wall covered with full bookshelves. However there was a small section of wall completely void of bookshelves where three banners hung. The first was obvious as it was Twilight's cutie mark on a field of lavender; it was his banner. The second was the familiar six gems arranged in a pentagonal pattern that was often used to symbolize the elements of harmony on a white field (The Elements of Harmony banners had been given to all the bearers but other than Twilight only Rarity and, interestingly enough, Rainbow Dash had actually put them up). The third banner was one that would be known by few outside of the nobility; it was three six-pointed stars, one magenta, one cyan and one yellow, and a yellow crown arranged in a triangular shape on a indigo field, with a – recently added – yellow crown above; it was the banner of House Twilight.

Twilight still missed the Ponyville library or “Golden Oaks Library” as he had heard some of the residents call it. Unfortunately, it was now a crater in the centre of Ponyville after Tirek had destroyed it. The crystal castle was great, but so often Twilight missed the simple life of a librarian.

Of course none of this was relevant at this point and he was most likely procrastinating from the important issues. In a very un-Twilight way, even five hours after his transformation into a stallion, he still didn't have a clue why it had even happened.

“I know what I am going to do” the lavender alicorn with a renewed confidence.

“What?”

“Even if the problem has no solution, it would be irresponsible to not find the cause.”

“Oh”

“Now the spell that caused this to happen was not a transfiguration spell so it technically shouldn't have been able to cause this transformation”

“But the spell that turned you into an alicorn was not a transfiguration spell either”

“No it was not; a very astute observation by you. I suspect that that spell used an indirect method for transformation. If a transfiguration spell was to be used it would have given me wings but not necessarily the pegasus magic associated with it. Rather this spell gave me the magic of an alicorn and let the magic transform my body into the shape of an alicorn. And that is not even mentioning that that spell also invoked the Elements of Harmony which seem to work under entirely different constrains than normal magic; they can cast a effective disjunction spell without exploding in our faces”

“Maybe this spell did something similar”

“No, it couldn't. The difference between the two sexes magically is less than the standard deviation of either; this method of transformation would not only require strong distinct differences in magical balance, but would require physical traits to be dependent upon said magical balance.”

“Well what was this spell supposed to do? I would think that would be a good place to start.”

“Yes, it is. This spell was a spell made by Starswirl the Bearded designed to clean all the body's ley gates of blockage and balancing the internal magic of the caster greatly increasing the caster's efficiency in casting future spells. However I haven't the slightest clue as to how this could translate into sex modification.”

And not once over the next three hours did this last statement diverge from the truth. Exhausted and discouraged Equestria's newest prince crawled dejectedly into his bed.

Well, here I am again. I have started renewing this fic. I have changed a number of things from older drafts:
1. This is now post s4 rather than post s3, no point making a contrived excuse as to why Twilight has a castle when I can just use the castle she gets in canon
2. I changed the letter significantly adding a lot more information, while I always had an idea of why Twilight couldn't become a mare again, I always had Celestia be incredibly vague and unreasonable
3. A number of rephrasing and assorted editing to make the story flow better

I will be editing all the chapters before continuing this story, I already have chapter 2 and 3 ready for re-release.