I was still a bit giddy when I opened the front door, so giddy that I let the fez Pinkie had given to me fall off my head. I grabbed it off the floor and put it on the hat rack.
“You should have been there, Twilight. Pinkie really went all out on this one. There was cherry punch and orange punch; really every punch, except for apple. I still don't understand what was so important about studying the—”
I held my breath. Something was odd. There was no light burning in the study, or anywhere else for that matter. Twilight never went to sleep this early when she got caught up in her books.
“Twilight?” I called out into the empty room before me, but she didn't reply. Nothing did. All I heard were my own steps across the hardwood floor.
She must have gone out to get something for her studies. I hadn't really understood what they were about, but it sounded too difficult and complex for me to understand anyway. And like something she'd need additional instruments for. She surely left a note somewhere, telling me where she had gone. I'd only have to find it.
It didn't take me long at all; it was lying right in the middle of her writing desk. I put my lantern down and started to read:
Spike!
I don't have much time, so I need you to listen very carefully:
Something went wrong with the spell. It must have been because I looked into the mirror when I changed Fluttershy back. I can already feel its effects, and I don't know if there is anything I can do to slow it down.
It is crucial that you inform Princess Celestia asap! She'll have an idea what to do and how to turn me back. I don't think there is anypony else who can learn the spell fast enough. I wish we could trust Rarity to perform it, but with all the damage it has done, I don't want to put her in harm's way.
Do not allow her to perform the spell, Spike! Under no circumstances whatsoever!
Until then, all you can do to help me is to stall the transformation by pinching off my body's internal magicstream. There is a simple potion to coagulate some of the magic flowing through me. We should have all the ingredients needed in the lab and in the kitchen.
Even if it looks like I'm fully transformed, you still need to administer the potion. We can't be sure how thoroughly this spell permeates through my theurgycocytes. For all we know, the spell could be
No, I'm certain the transformation is completely reversible.
Still, please administer it anyway. Just in case.
For the basis you will need some aluminum sulfate. There is a flask full of it down in the laboratory, on the topmost middle drawer next to my laboratory bench. You need to dissolve it in water. About 50 mg in 100 ml. Then you need to add some more stuff to it.
Juniper and butterbur. Both should be in the cubpoard above the sink. Mince them and put them in.
Lastly use lime. Juicest take some from the fruit basket. If their are no limes, use lemmons.
Squeeze them and ad three drops.
Apple Appley the potion in the mouth
Spike , dont be scared
I no u can do
Then it broke off. At the end she had pierced a hole through the paper and torn it down to the bottom of the page. There was a scrape from her quill on the desk, running all the way down to the edge.
I didn't care what she said. If she was in pain, I had to find her before anything else. But with the little light coming from my lantern I could barely see the claw in front of my face. I walked over to the window to part the curtains and let in a little moonlight.
Only they weren't curtains hanging from the pole. I had misidentified the wings in front of me, now unfolding to double their size, as a pair of eyes stared at me from in between them. She let out a loud shriek as soon as she saw me.
I tried my best to remain calm. “Twilight, it's me,” I said. “Don't you recognize me?”
Nothing told me she did. With another shriek, she pounced at me, only barely missing my head.
I jumped back on my feet and quickly picked up my dropped lantern, desperately hoping I hadn't lost sight of her. With the moonlight coming from the window, I could clearly see her standing by the door, cutting off my only means of escape.
“Twilight, please calm down,” I tried again. “We'll turn you back right away. I promise.”
I was reassured by the way she was standing there, on all four of her hooves, deeply lost in thought. If it wasn't for the giant bat wings and the fangs in her mouth, I couldn't have told she'd been transformed at all.
She didn't move in the slightest. “Did you calm down because I told you to?” I asked. Still no reaction from her, but that was as good as a yes. “Thanks. I need you to stay here and wait until I get Princess Celestia and the others. Would you please do that?”
It was already too late when I realized what she was actually doing. The spell she had been charging was now ready and she hauled the magic bolt straight at me. I had no chance to dodge it. The magical surge of energy enveloped me from top to bottom and quickly started coursing through my body with a burning sensation. Out of the corner of my eye I could see how Twilight took to the air again and pushed open the window.
Luckily, the spell didn't take any effect; freshly transformed unicorn bat ponies probably couldn't use their magic very well. The burning stopped as quickly as it had begun and immediately I was up and by the window, looking after her. She did not fly towards Sweet Apple Acres like I had expected. In fact, she was heading in the completely opposite direction, towards Carousel Boutique.
Rarity! I had to run over there and save her; save both of them.
There was no time to put together the potion. There wasn't even enough time to sit down and write a letter to Princess Celestia. I decided to grab a piece of paper and a writing quill and hope I'd be able to draw one up on my way over there.
But first I needed something to defend myself with. If Twilight would pounce at Rarity the same way she had at me and I wasn't able to stop her... I don't think I could ever forgive myself. And the only thing I knew with which to stop a vampire bat effectively was garlic.
I rushed down to the pantry and went through the shelves as quickly as I could. I looked everywhere, up and down, left and right, front and back, but the garlic had vanished from the library completely. All I could find were a couple of bags of fruit which I tossed to the space by the door.
Gradually, it got harder and harder to concentrate. I caught myself glimpsing at a bag of apples right next to me from time to time. The smell was staggering; so incredibly juicy and inviting.
There was no point in continuing with such a huge distraction right in my face. I first needed to have just one apple to satisfy my hunger before I could do anything else; whatever I was looking for could wait for a moment.
I took one of the shiny red apples out of the bag, rammed my fangs into it all the way to the core, and started sucking. I sucked and sucked until it was sucked completely dry.
I felt much better afterwards; refreshed and recharged to new heights of energy. I wanted another one. I needed another one. And surely, having another one wouldn't hurt.
After sucking the second, I sucked the third, and then the fourth. I kept taking more and more from the bag until it was completely empty. But I still needed more. And I knew where to get them from: Sweet Apple Acres.
I would fly there real quick and grab a couple of them right off the apple try. I could even move into the orchard. Over there was paradise. There I would have all the apples I could ever eat.
Author's Note
1500 words is a very tight word limit 
Don't forget to check out the other works of the Writer's Training Grounds #001: "Bats!", for more Flutterbatial goodness.