"Expecto Patronum!"
"Is that really necessary?"
"You know, now that you mention it, I think I could cast it wordlessly."
"Are you being deliberately obtuse?"
"No," said Harry. "Wouldn't want a stray Killing Curse to escape the practice zone and keep going until it hits something." He looked at his patronus. "Stand guard and intercept any killing curses that don't hit the beehives."
The silver humanoid snapped a solute, then began marching around the cave's perimeter. The movement wasn't necessary, Harry knew. It would teleport into place if it had to. But it gave the impression of guarding.
Tom Riddle sighed. "I do not think you are getting into the spirit of this spell. At all. Caring about collateral damage means you are not truly indifferent."
They were both currently human, as they would have to be to exchange finger gestures. They stood in a deep, dank cavern now lit by Patronus light. Not their ordinary hideout, but part of the same cave network. There wasn't a crystal in sight.
Cold, dark places worked wonders for learning the Killing Curse, apparently. Pictures of bitter enemies were also considered helpful, and students were often told, as homework, to brood on what they would do if they got their hands on the one they hate most.
Harry hadn't done any of that, and he didn't plan to. In the first place, he only considered it possible that he could learn the Killing Curse for two reasons.
One, he believed he would be able to achieve an apathetic mindset towards non-sapient creatures. He has squished literal insects before, like mosquitos, and this should be no different.
Second, Harry is almost certain that back when he killed the troll, his hatred at the time could have fueled a killing curse. He would have used the killing curse if he knew the gestures and had the power. He'd certainly had the right mindset.
If it's not sapient, it's not morally important. He doesn't care, and that indifference might be enough to cast Avada Kedavra at a beehive, even while maintaining a Patronus charm. And if apathy doesn't work, he'll try hatred, just to be honest.
If he has his Patronus charm active, he can tell it to intercept any stray green bolts before they strike something sapient. Assuming he can use both magics at the same time. They might be too incompatible, one based on caring, the other based on apathy.
"I'll dispel my Patronus if I feel like it's interfering," Harry assured his teacher.
If that conflict did happen, he might have to work his way up. Even if he can't do both spells simultaneously at the start, he might be able to do it eventually. It would mean relying on Tom to intercept stray curses until then.
Not with his body, of course. Killing curses can be blocked by anything that has a brain. Simply levitate a living creature into the path of the curse to neutralise the green spell.
It's hardly practical to do that in live combat when dodging is more efficient and reliable, but it can be done in a classroom setting. If Harry misses the beehives, Tom would simply move one into place. The bees are fully sealed inside glass cases for that reason.
"Do you think I'll have enough magic for the curse?" Harry asked. "Wasn't it a fifth-year spell in terms of power?"
"For now, as you learn the gestures and pronunciation, that is a good thing. Once you have memorised the preliminary parts of the spell, I have a solution to the problem of power."
Harry found himself surprised that those words did not sound ominous.
The words to the Killing Curse had to be exactly precise, like the Aguamenti spell. It involved finger gestures and wand movements, but those didn't have to be precise. And of course, it required magical power. Harry was almost disturbed at how the spell mirrored the Patronus Charm so invertedly. He wondered if he wasn't the first wizard in history to cast the Patronus Charm version 2.0 and block a Killing Curse after all. Maybe the original creators – the Atlanteans? – had known.
Once Harry had gotten the spell's prerequisites exactly correct ten times in a row, it was Tom's turn to learn the preliminary parts of the Patronus charm, as per their agreement. Harry had decided, in the end, to keep Hermione's Patronus lesson separate.
Tom took little time to get good and consistent, despite the difficulty. A result of his experience learning countless difficult spells, no doubt.
Then they shifted back to Harry.
First, Harry proved that the brief break hadn't interfered with his own consistency. Then Tom Riddle drew forth a wand of knobbed wood – the Elder Wand, which Harry had completely forgotten about.
"Before I grant you access to this powerful artifact," said Tom Riddle, "promise in Parseltongue that you will not attempt to take it from me, now or ever."
"Um..." said Harry, "why are you even letting me use it in the first place?"
"You might eventually wield different world-saving magics than I, which the Elder Wand might have to make as powerful as possible. I suspect that a centaur might tell either of us that we both must learn how to wield it, in order to prevent the world's end in the future. We are both Tom Riddle, so it might be convinced to respond equally to both our hands, and consider that we both vanquished its former master. But I will decide who uses it at any given time. Is that understood?"
Harry nodded.
"Repeat the full oath in Parseltongue. Say that you will not take it from me, that if circumstances force us apart while you are using it, you will return it as soon as you are able, and that you will not attempt to bypass this oath through any trickery."
Harry hissed the words.
And Tom Riddle levitated the Elder Wand to Harry Potter, bringing their magics close enough to activate the sense of doom, since they were both currently human.
The resonance wasn't nearly as strong as it had been on the other side, possibly because Equestria's natural harmony lessened their own anti-harmony. It was now more of a minor annoyance than a blaring alarm.
Even an object being levitated by Tom's magic mere inches above Harry's arm didn't produce anything more than a tickle. Then the wand was allowed to drop, and Harry accepted the wood in his right hand, his left already holding his personal wand. He was using non-dominant casting to keep the Patronus Charm as weak as possible without winking out, which left his dominant hand empty and available.
"First, we shall test your power," said Tom. A small cage containing a single bee was levitated to Harry's feet.
Harry picked it up, noticing the broken wing.
"Cast-"
"Episkey," Harry said, staggering only slightly at the amount of power it had taken to mend the gossamer wing. As a pony, that wouldn't have winded him.
"Very good. As I suspected, you should have enough strength to cast the Killing Curse when you are wielding the Elder Wand." Tom sighed dramatically. "Now for the tedious part. Aim your wand at the bees and begin your attempts. And do please conclude that your Patronus cannot be active in a timely manner."
Tom was right. Harry quickly concluded that he wouldn't be able to do both spells simultaneously. Not yet. Not while he was learning.
The Killing Curse felt wrong to cast. Actually, that didn't describe it accurately. The best way to describe the feeling of a failed Killing Curse was that the spell didn't feel right, which disturbed him. It was like the spell was telling him that it would feel right if he succeeded. Schadenfreude was the technical term that came to mind, the feeling of joy at the destruction of enemies. That emotion is, unfortunately, a part of human nature.
Harry wondered if wizards normally feel schadenfreude after using a Killing Curse, or if it was because he was trying to use apathy instead of hatred. Or maybe he was remembering Tom Riddle's neural patterns of anticipated joy for getting rid of an annoyance, triggered by speaking the words 'Avada Kedavra'.
The normal learning path – the only learning path, as far as all other wizards are concerned – is to use hatred to fuel a killing curse. Even if a wizard does manage to reach the second level of apathy-fueled killing, they typically used hatred so much that they became empty over time. Harry wondered if he was the first wizard in the world who was trying to skip hatred entirely. He wondered if it was even possible to skip hatred.
If not, he might not ever cast it. So far, prospects weren't looking good even after dismissing his Patronus Charm.
"You know," Harry said eventually. "Excluding the troll, all the things I've killed in my life have been much smaller than this. Mosquitos, at the largest. Maybe bees are too large, or I'm implicitly caring about their roles as pollinators?"
"Nothing smaller would block a killing curse."
"Are you sure?" Harry asked. "Has anyone tried casting it into an anthill? Or a massive cloud of gnats?"
And that is how the learning session derailed into a scientific endeavour. Just how large did a creature have to be to block a Killing Curse?
First, Harry Potter turned back into Silver Wing, so their magics wouldn't resonate out of control. Then he cast his Patronus, again with his left hoof, and readied it to intercept stray Curses.
Mr. Tome went through the tedious process of buying artificial anthills from various petshops and lining them up in front of the beehives, at enough of a distance that Harry could observe from the sidelines if the Curse was intercepted by the anthills. It was winter, so they couldn't exactly go through the normal process of gathering insects from the wild.
It turned out that ants could intercept Killing Curses. The only problem was precision. They had to line up multiple hills in a row until they were lucky enough that the curse exactly collided with one of the ants. Even aiming at concentrated pockets of ants didn't guarantee that the Curse would hit one. Not unless Mr. Tome used his own magic to clump them together so tightly that there were no gaps in the writhing mass of legs and skittering.
From this, they both concluded that the Killing Curse, despite its large outward appearance, was ridiculously precise. Like a perfectly thin line segment extending straight from the wand at the moment of casting.
Armed with this new knowledge, Mr. Tome used Silver's bits to buy more anthills. He had provided the beehives himself, but he wasn't going to indulge Silver's additional requests out of his own pocket.
He put all the anthills in a line in front of the beehives, then decided to stay as Riddle Tome, just in case something happened and he had to use his magic in a way that would collide with Silver's. Silver became Harry Potter, and they went back to the lesson.
They went back to the failing lesson.
"Maybe if you tinted the glass so I didn't see the ants, I would care less?"
This was done, with a look of extreme distaste on Riddle Tome's face.
A look that vanished when Harry Potter successfully cast his first Killing Curse.
It felt like it should have been more dramatic. Like there should have been some sort of emotional climax after a long and grueling journey.
Nope. Just a green flash disappearing behind tinted glass.
Turns out that cold, clinical understanding of the human mind is what it took to not care.
Visual cues are a big part of the human emotional system. And so is tactile information. And distance. And attachment. More of Milgram's test subjects made it to the XXX (deadly) electric shock when they couldn't see the actor they thought they were shocking.
Killing a person with a knife, with your own hands, is intense compared to shooting a gun. Shooting a gun at a visible target is intense compared to pressing a button. You can still feel the recoil and hear the pained shout with a gun. Pressing a button when you can still see the target is more intense than if the other person is out of sight, and therefore out of mind.
What Harry had just done to some random ant he couldn't see had felt like pressing a button.
"This is the part that I tell you all those ants were transfigured from live ponies."
"Parseltongue or you're lying."
"After months in my employer's presence, I've come to prefer the term 'joking'."
"It wasn't funny."
Riddle Tome grinned. "I thought it was extremely funny. As royal fool, it is my privilege to make jokes, even if others consider them to be in poor taste. I had been sorely tempted to give Ms. Memory a Troll for her Defense grade, though I ultimately decided that the Defense Professor was too dignified for it."
"As your Patronus Charm mentor, it's my privilege to tell you that any happiness you felt from those jokes won't work to fuel a Patronus charm."
"You are not my mentor yet," warned the wizard. "Replicate your results. Otherwise I will consider your earlier success a fluke."
This was easier said than done. First, Harry had to recover enough magic to cast it again. Then, as he stared down the line of anthills...
"Did you have to make that joke?"
To Tom's clear annoyance, it took a Parseltongue reassurance that he had indeed been joking, that the ants and bees were indeed ants and bees, for Harry to be able to cast the Killing Curse again.
Then Harry asked if there were any spells that gave the binary result of 'sapient' or 'non-sapient', and Tom taught him that one as well, again reassuring him in Parseltongue that the spell didn't do anything else.
Because not all past wizards had been ethical monsters. Just as there had been a wizard that had created House Elves, so too had there been a witch who was curious enough to wonder if Mermaids were really unintelligent beasts.
Once he got into the rhythm of casting the Killing Curse, once he could feel the emotional mechanisms for apathy in his own mind and body, he asked Tom to un-tint the glass. It took a few tries, but he eventually activated the feeling of apathy despite the ants being right in front of his eyes. Then he moved up to the bees. Then he became a pony and continued replicating his results.
And then, the really hard part. Now that he could do it as a pony, his attention wouldn't be occupied by outputting enough magical power to fuel the spell.
"Expecto Patronum."
But the first attempt failed. He had to remind himself of the value of sapient life, remind himself of all bright and beautiful things.
"Expecto Patronum! Stand guard. Be ready to intercept any killing curses."
The moonlight alicorn waved a winged salute, then began flying in a defense formation.
"Can you tint the glass again?"
Riddle Tome did this, even as he said, "You do realise you have already kept up your side of the bargain by the standards of my promise?"
Silver nodded. "I know. But... I feel like I should do this anyway. Or at least try. Outside a classroom setting like this one, I'm only ever going to cast a killing curse if I can use my Patronus charm to prevent collateral damage. This whole lesson is practically pointless if I can't cast a killing curse under real-life circumstances."
Riddle's eyebrows rose, but he stayed silent, watching as the alicorn with a white-glowing mane began trying and failing to cast a killing curse.
But he didn't fail for very long, this time.
Silver quickly realised that, in order to be both caring and apathetic, an obvious solution was Perfect Occlumency. His true, inner core would care, while his complete outer personality would maintain the neural patterns of apathy. Just like when he was rescuing Bellatrix Black.
That state of mind was enough to achieve the mental gymnastics required to cast a Killing Curse while maintaining a Patronus Charm.
The instant he succeeded, he felt another tingling on his cutie mark.
He looked back and saw that the number 5730 had been added to the bottom left corner of the picture. Three corners of the image were occupied now. The letter "C" in the top right, the number "6" in the top left, and the number "5730" in the bottom left.
Harry understood instantly.
"That number looks familiar," said Riddle Tome.
"It's-"
"No," said Riddle. "I shall figure it out on my own. A Slytherin should not have to remind a Ravenclaw of that."
There was a pause.
Then, "Ah! Yes. Carbon dating. Five thousand, seven-hundred and thirty years is the half-life of carbon."
"Yup," said Silver. "Do you want to figure out the meaning on your own too?"
"Not quite. I would like to attempt the problem before you say the answer," said the thestral. "Meaning is subjective, unlike fact. There are multiple potentially correct answers. My own perspective suggests that the half-life of carbon, which is sometimes called the element of life, is meant to remind you that all life will inevitably decay and die. Your earning that number after learning the Killing Curse lends to this interpretation."
"Almost," said Silver. "I think it's meant to remind me that we're on a timetable. Life will inevitably die if I don't do something about it."
Like, say, by trying to get Lord Voldemort to the point that he doesn't want to kill people all the time. Unfortunately...
"No."
"What? But you promised-"
"To put forth a genuine effort at learning the Patronus Charm if you learned the Killing Curse," he quoted. "I did not agree the effort would be my main priority. I did not agree to go immediately to extreme lengths if my early attempts fail." The thestral's slitted eyes narrowed darkly. "I did not agree to take happiness lessons. I will look into the matter independently. If I feel as though I am making no progress, then I might ask for aid. As it stands, I cannot imagine you have already implemented your own best efforts to the task of devising a good lesson. I suggest you take the time necessary to craft it in such a way that it would not annoy me in any way, shape, or form."