Spring looked good on the Canterlot Castle Gardens, though the same could probably be said for any season. Lush grass, flourishing flowers and full green trees greeted Celestia and Twilight as they walked the trail on their way to the site of the morning's lesson—or at least that's what Celestia had told her student.
Nopony ever said that teaching was easy, nor had Celestia gotten into it just to pass the time. She'd known from the start that it would be a lot of work. And also that it would be worth it in the long run.
Something she'd come to realise as the years passed though, was that some lessons were more difficult to teach than others. Even before she'd picked Twilight up that morning, she'd known that today's was going to be one of those.
“So, today's lesson is outdoors?”
Celestia looked down to find Twilight smiling back at her attentively. Twilight wasn't exactly a 'little' filly any more. She was in her last year of school and she looked the part—just a few inches short of becoming a full-grown mare. Her understanding of magic had grown accordingly, as had her powers of observation, and she'd become a formidable sorceress in her own right. As loathe as Celestia was to admit it, Twilight was ready to learn this lesson.
“Indeed,” said Celestia, returning the smile. The problem was, Celestia wasn't sure if she was ready to teach this lesson.
Twilight grunted, satisfied with the simple response. She faced front again, but didn't give up trying to make conversation. “Do you mind if I ask where we're headed?”
Celestia's expression didn't change. Years of practising composure had taught her to hide signs of uncertainty. But in all honesty, she didn't know. The Gardens in general provided the ideal location for her lesson—being so vibrant and full of life—but she hadn't picked an exact spot. It was an impossible choice.
“Somewhere out of the way,” said Celestia, half to herself. She turned left, leaving the main trail and leading Twilight deeper into the Gardens. Nopony would be out there at this time in the morning, save for the groundskeeper, and Celestia had deliberately given him the morning off.
Twilight's smile wavered. She failed to hide a curious glance at Celestia, who made no comment. They walked in silence for another half a minute before they came across a patch of orange in a small clearing. “Oh,” said Twilight, slowing.
The fox started at the sound, took one look at the two ponies and dashed off into the brush. It left behind what would doubtless have been its dinner—a motionless little bird which was a mess of feathers flecked with red. Twilight and Celestia came to a stop a hoof's distance from the tiny creature.
Twilight leant in a little, her features touched with sadness. “Poor thing,” she said.
“It's dead?”
Twilight closed her eyes and took a breath—a motion Celestia had taught her to help focus her magical senses. When she opened them again, she sighed and gave a solemn nod.
Celestia didn't say anything. She looked down at the little bird, considering it. She'd been looking for the right place to teach her lesson... Finding this place may have just been a coincidence, or it might have been nature's way of giving her a push in the right direction. Stranger things had happened. Either way, the opportunity that had presented itself was too good to ignore.
“This isn't your first time seeing this kind of thing, is it?” she asked, facing Twilight.
Twilight shook her head. “No. I used to have a cat—Salem. He would always bring home birds or mice like this.” She paused, and a smile crossed her face. “I don't think he ever got it through his head that ponies don't eat meat.”
Celestia cracked a smile too. “I don't think I've ever seen a cat at your home.”
“He died a couple of years ago,” Twilight said.
“Ah. My apologies.”
“It's alright,” said Twilight, shaking her head. “I was devastated at the time, of course—I'd never lost anyone like that before. It was his time, there was nothing I could've done, but it took me a while to get that through my head. I guess I've grown up a little now.” She flashed Celestia a smile that didn't reach her eyes.
“...What if there was?” Celestia found herself saying.
Twilight blinked at her. “I beg your pardon?”
“You're right that death—as a rule of nature—is not something we can change or prevent.” Celestia gave her student a level look. “What if I told you that you could break that rule?”
If Twilight was following her line of thought, the blank look on her face didn't show it. “Is this... a test?” she guessed.
“Neigh,” said Celestia, shaking her head. “Just a simple question.”
Twilight stared at her for a long moment. “Well, then... I suppose I'd say that it was highly unlikely. Magic and medical practice can cure illness and generally help ponies live longer and healthier lives, but in the end, everything dies--” Her voice wavered on the last couple of words. She visibly tried to swallow her hesitation. “--Doesn't it?”
“So you have been taught,” said Celestia, nodding to herself more than Twilight.
She paused to give it a last bit of thought before crossing a line she knew she could never come back from. Twilight was young—enough so to still be open-minded and adaptable—and she was also incredibly intelligent and wise beyond her years. If this lesson was ever going to be taught, now was the best time to do it. That wasn't in question. What was in question was whether Celestia should be teaching Twilight this lesson, or protecting her from it.
A long look into her student's inquisitive eyes gave her the answer she needed, if not the one she hoped for.
“Yes,” Celestia sighed. “You should know the truth, Twilight.”
Twilight gulped. “Yes?” she prompted.
Celestia fixed her with another stare. “You must promise me that what I am about to show you, you will never show to anyone else.”
“Of course,” said Twilight, without thinking. A long moment of silence passed between them. Then Twilight nodded and repeated herself, with meaning behind it this time. “I promise.”
Celestia took a steadying breath through her nose and let it out slowly. With a satisfied nod, she turned her attention again to the dead bird. Twilight watched her avidly.
An effort of will caused golden light to emanate from her horn, as she focused on the spell. It'd been a long time since she'd last drawn upon it, but it came back to her as easily as a practised motion. That was a little unnerving, to say the least. She pushed the distracting thought to one side and focused on what she wanted to achieve.
The light spread out across the grass at her hooves, until it flowed over the bird like a blanket of sunlight. Celestia closed her eyes. There was a faint rush of power, and then silence.
For a fleeting moment, Celestia thought that the spell might actually have failed. That moment lasted only until she heard a gasp from Twilight's lips, and the rapid fluttering of tiny wings. She opened her eyes to find the little bird—no longer dead, but alive and hyper-animated—hovering a hoof's distance away from her nose.
It stayed there, staring at her with tiny, unreadable eyes. As ridiculous as it felt, Celestia held the bird's stare. They stayed there for what might've been half a minute before the bird chirped, spun in place and darted off into a nearby tree.
“Inconceivable...” Twilight breathed, watching the little thing go. Her eyes were wide with wonderment, and her mouth was part open. After a beat, she turned to Celestia and started shaking her head in awe and disbelief. “I-I don't understand. It was dead. I felt it!”
“And now?” said Celestia. She kept her expression neutral, though her heart was hammering in her chest. It wasn't all because of the strain of the spell, either.
Twilight hesitated, then closed her eyes and took a breath as she had before. The little bird flitted overhead. She opened her eyes and shook her head again. “It's... alive. As alive as you or I.”
Celestia nodded.
“How?”
Celestia didn't answer right away. She took a few, slow steps to one side and touched a hoof to the trunk of a tree, choosing her words carefully. “All living things are born with magic inside of them,” she said. “Unicorns, Pegasi, Earth Ponies. Animals, flowers, trees. Everything.” She paused and lowered her eyes. “When they die, that magic leaves their body—leaves behind only a corpse.”
Twilight nodded along, clearly remembering it from her previous lessons. “That's how I could sense that the bird was dead, yes. But once that magic is gone, it can't just be brought back. It's gone forever--” She bit her lip. “--Isn't it?”
“You are correct,” said Celestia. “And yet, for life to be restored to the bird, the same volume of magic that was lost had to be returned to the vessel.” She fought back the urge to lick her lips, which had gone dry. “That magic has to come from somewhere.”
Her eyes traced unconsciously to the spot upon which the dead bird had lain. Twilight followed her gaze, to what had moments before been a patch of vibrant, lush green grass.
And which was now a yellowing circle of withering life.
“...It's dying,” said Twilight. It didn't take her long to put two and two together. It never did. “You took the life from the grass and... put it in the bird?” Celestia took the emphasis on the word 'put' as confusion rather than disgust.
“Along with a small amount of my own magic, yes.” Celestia raised a hoof to stop the suddenly growing look of worry in Twilight's eyes. “Do not worry. One such spell will not affect me in any noticeable sense. Most of the energy came from the grass itself.”
The concern faded from Twilight's expression as quickly as it had appeared. After a moment, it was replaced again with sheer wonderment. “This is revolutionary, princess. If I understand correctly—if it's possible to literally move magic from one vessel to another... Goodness, the possibilities are unfathomable--”
“Twilight,” Celestia said. “I have your promise that you will not show this magic to anyone else.”
The building excitement vanished as Twilight remembered her own promise. “But why?” she asked. “If you—if we—can actually bring animals back to life with magic, we can do it on a larger scale for ponies. And I haven't even begun to think of what else might be possible using this kind of energy.” She fixed Celestia with a solemn look. “Don't we have responsibility to use it?”
“That is a philosophical question, Twilight,” said Celestia, shaking her head. “Personally, I have always considered it my responsibility not to use this kind of magic, because of the cost it incurs.”
Twilight just stared at her, not understanding. Celestia faced the tree trunk for a moment and racked her mind. How to make her understand?
It came to her a moment later, and she lifted her chin a little without facing Twilight. “Let me ask you a hypothetical question, my student. If I were to die--” She felt more than she saw the colour drain from Twilight's face.
“Princess--”
“--If I were to die,” Celestia continued, ignoring it, “and you had the power to restore my life at the cost... say, of the life of this Garden... would you do it?”
“O-Of course I would!” said Twilight. Once again, no pause for thought.
“Why?”
Twilight licked her lips nervously. “Because I care about you,” she said, in a quieter voice. “There are hundreds of gardens in the world, and this one could always be replanted in time. But you--” She shook her head. “--There's only one of you. And you could never be replaced.”
Celestia nodded along with the reasoning, taking it in. Then she gestured up into the branches of the tree. “And what of that squirrel?” she said, picking out the tuft of brown fur hiding amongst the leaves. “If I asked him the same question, what do you think he would say?”
“Sorry, I don't speak Equestrian?” Twilight guessed. Celestia shot her a look, but she was shrinking even before it arrived. Twilight gulped and shook her head. “I-I don't know.”
Celestia held her gaze for a moment more, before looking back up to watch the squirrel hop between the branches. “This creature doesn't know me,” she said. “To him, my life is worth just as much as that of any other animal. But this Garden, this tree, even--” She placed a hoof on its trunk again. “--is so much more than that. It is his home.”
Twilight didn't say anything, which Celestia took to be a sign that she was listening. She thought for a beat before continuing. “The tree keeps him safe from predators and cold weather. The garden keeps him fed. The stream a short ways down the trail provides him with water. It does all of this for him--” She found a smile as she made out a smaller tuft of fur higher up on the branches. “--and for his children.”
Finally, she turned to meet Twilight's eye again. She wore a thoughtful frown, but she wasn't looking at Celestia. She was staring at the now blackening patch of grass in the middle of the clearing.
Celestia could picture some of her past students coming out with something like 'But there are dozens of other gardens around Canterlot, we could just find him another home!' But Twilight knew that Celestia wasn't talking about this tree, or this garden, alone. She was talking about what it represented. Something that meant nothing at all to one observer, and the entire world to another...
“There's a cost,” Twilight said quietly. “A choice.”
“Indeed,” said Celestia. “Every time we use this magic, we make a choice. A choice between the life of something we love, and something loved equally by somepony else.”
Twilight nodded, her face grim. “It's selfish magic. Dark magic.”
“It can be, yes. Though it is worth noting that it would work in the opposite way as well—I could just as easily sacrifice myself to restore the life to a dying forest. Doing so would, of course, cost me my own life.”
Twilight thought it over for a moment longer, before meeting Celestia's eye. “That's an impossible choice.”
“And not ours to make,” said Celestia. She found herself turning her head to one side, to stare out into the gardens, focused on nothing. “Or, at least, that is what I have told myself these many years...”
A long moment passed in silence. Whether or not that philosophy was 'right' or not—whatever that meant—was something she'd pondered for centuries. With scarce few ponies with whom to discuss the matter, the best answer she'd been able to come up with was that it was impossible to know for sure.
A gentle touch on her chest brought her back to reality. Twilight had walked over and arched her back against Celestia's chest in a one-sided neck hug. “Thank you for showing me this, princess,” she said. “I imagine it would've been a lot easier for you to keep it from me.”
Celestia gulped and turned her gaze to the sky, fighting back a sudden uncomfortable pain in her nose that was making her eyes start to water. Once she'd gotten it under control, she leant down to return the hug. “Yes, I imagine that it would have. Though you would've discovered this on your own, sooner or later.”
They held the hug for a long moment. For once, Twilight was the one to break it. She stepped back and looked at Celestia with searching eyes. “Really?”
Celestia nodded. “When the temptation arose, yes. You've already displayed an innate talent for making things happen when your emotions are running high. I have no delusions that if you were ever put in that situation—where you would do anything, sacrifice anything--” She broke off, shaking her head. She didn't need to finish that sentence. “I teach you now so that when the time comes, you'll know and understand the price, ahead of time.”
With another gulp, Twilight turned to stare at the patch of grass—the size of a pony's noon shadow—which was now completely blackened and dead. “Yeah,” she breathed. “I don't think that's the kind of thing I'd have liked to find out on my own.”
“Indeed...”
“Will that ever grow back?” Twilight asked.
“Would the bird have?” Celestia answered.
Twilight said nothing. It was answer enough.
“I do not know exactly what will happen to this place,” said Celestia, staring. “I have not practised this magic in a very long time. It has never been properly studied.”
Twilight nodded. “Maybe it's best that way,” she said, quietly enough that she was probably talking to herself.
“Perhaps it is,” Celestia found herself saying. She watched the eyesore of the dead grass for maybe five seconds more, before turning away, back towards the trail. “Come, Twilight. We should get back to the castle.”
“Okay.”
The two fell in together and started walking away. Just before they left the clearing behind for good, Celestia turned to her student one last time. “And Twilight,” she said.
“Yes, princess?”
“There will be no report on today's lesson.”
Twilight paused for a long moment before her reply. This time, she thought about it.
“...Yes, princess.”
* * *
Author's Note
Well, there you go! Something a little bit darker than I usually do, but it was interesting to write and think about (exploring magic is something I've always been interested in with my writing). Only dipping my toes into the dark side though, so those of you who prefer my cute fluffy stuff, don't worry! 
Cheers for reading! 
EDIT 20/10/15: Made some very minor changes so it makes more sense, now that I've had some time to ponder how it would all work.
