"You have no idea what I had to become to overcome you."

Surrounded by a thick cloud of smoke that only the Breezies could fight for long, the trip up Hark Mountain wasn’t exactly scenic.

“How’re we even going to find the dragon?” Rainbow Dash complained. “He could be right in front of us, and we wouldn’t even know it!”

“Oh, why did you have to say that?” Rarity said.

“Niddhog could have attacked us any time when we were recovering,” Twilight pointed out. “I don’t think an eye poke took him a week to heal or that it scared him off. I bet he’s at the top of the mountain sleeping on a pile of gold and jewels, waiting for us to challenge him.”

“Why would he do that?” Applejack said.

“I tricked him. I took some inspiration from Rarity, of course.”

“But the dragon is, is malin come un singe,” Rarity said.

“Smart can be a weakness,” Twilight said. “Dragons can learn, and they can learn the wrong lesson. I think I know what he’s thinking.”

“And you can still slay him, even then?” Rarity said, aghast.

“Mmhmm. Took some inspiration from Fluttershy, of course.”

“What would you have done if it hadn’t worked?” Fluttershy asked.

“I would have used my backup plan.”

“Your backup plan?” Pinkie Pie said.

“Yup. It’s even more dangerous than your sapling, so I’m glad I didn’t have to.”

“What could be more dangerous than that?” Fluttershy gasped.

“It’s really best if I didn’t say.”

“I have a question,” Applejack said. “How’re we gonna slay this dragon, anyway?”

“A primitive method.”

“Primitive?”

“The cool thing about markets is that you don’t have to understand anypony,” Twilight said. “You just give them what they ask for. But, you know, it’s actually pretty helpful to understand other ponies.”

“It would be nice,” Fluttershy said.

“Imagine if you didn’t have to ask what other ponies want,” Twilight said. “Imagine if you just knew. Because you knew them well enough.”

“Then you wouldn’t need to hear anypony,” Rarity said.

“Exactly. We talk to each other because we don’t already know everything about each other. But if we did, we wouldn’t need to. Remember the Prisoner’s Dilemma? How you both betrayed each other because it was both your dominant strategies? Well, suppose you could talk to each other, what would you say?”

“I’d promise not to betray her,” Applejack said.

“I’d threaten her,” Pinkie Pie said.

“Even if you could talk, there’s still some uncertainty,” Twilight said. “She could lie or change her mind. Suppose instead that you knew her. You knew what she would do. And suppose that she knew you and what you would do, and you knew this, and she knew this, and you knew that she knew this, and she knew that you knew that she knew this, and so on.”

“That would be like you having a little her in your mind speaking to you, and she would have a little you speaking in her mind,” Fluttershy said.

“The difference between the her in your head and the her out in real life is that the former never lies, never changes her mind. After all, she lives in your brain. She could hardly fool it!”

“So you can have a totally honest, trustworthy conversation with her, and she can have the same with you, and you both know it,” Rainbow Dash said. “That’s kind of bizarre.”

“How would a completely honest conversation go?” Twilight said. “Where you both know that you both know each other utterly?”

“Well, I’d tell her my dominant strategy is to betray her,” Rainbow Dash said.

“She’d tell you the same,” Pinkie Pie pointed out.

“Well, then I just won’t tell her!”

Twilight shook her head. “You can’t do that. She hears everything, whether you want to tell her or not. That’s how well she knows you.”

“Who is she?”

“Could be anyone,” Fluttershy. “And one day, everyone.”

“If she hears everything,” Rarity said, “then I have no choice but to tell her of my intention to betray her. But since she knows that, I know that she is going to betray me. How dare she? I’ll destroy—“

“It’s interesting, isn’t it?” Twilight said quickly. “Since you can’t help but broadcast your intention to betray her, she will certainly betray you, and you end up both betraying each other. It’s utterly self-defeating.”

“It’s better than getting a rump thump!” Rainbow Dash said. “Nothing’s worse than a butt putt!”

“…Okay. So betraying her doesn’t seem to work. What else could you do?”

“You could, um, not betray her,” Fluttershy said.

“Fat chance!” Rarity said. “That mare never misses a chance to betray others!”

Pinkie Pie frowned. “But she has to tell you that, remember? You know. So then you’ll just betray her.”

“That’s right,” Applejack said. “She can’t possibly benefit from betraying us when we don’t betray her because we’ll know about it and just betray her back.”

“So if you choose not to betray her, what’s her best move?” Twilight prompted.

“It’s to…not betray us,” Applejack said. “Well, I’m more surprised than a rattlesnake meeting a coral snake with a pair of maracas! It’s completely different from the Prisoner’s Dilemma from before!”

“If we don’t betray her, her best move is to not betray us,” Pinkie Pie said excitedly. “And if she doesn’t betray us, our best move is to—“

“Betray her,” Rarity said. Everypony looked at her. “What? I’m not letting that two-timing tart get away with ratting me out!”

Fluttershy winced. “You really shouldn’t say that about rats.”

“So as you can see,” Twilight said, “in the first Prisoner’s Dilemma where you didn’t understand each other, you were choosing whether  you would betray your confederate or hold your silence depending on what they did, and in both cases betraying them made you better off. But in this new kind of Prisoner’s Dilemma, your confederate’s best strategy is to choose whatever you choose. So your choice is: do I want us both to betray each other, or do I want us both to remain silent?”

“This is great!” Rainbow Dash said. “I was so worried I was going to be the victim of a seat beat!”

“Are you done with those?”

“Uh, well, I have one that kind of rhymes with thrash—“

“It’s a nice idea,” Applejack said. “I’m not sure how practical it is. Ponies don’t know each other that well.”

“Economists have been using exactly this idea for centuries. It’s how we fought King Sombra. Or rather, you could say it’s what motivated us to fight him at all. You could even say that this idea is the foundational idea of economics.”

“What idea? It’s a strategy.”

Twilight shook her head. “In order for this strategy to work, both ponies have to know that both ponies know that both ponies know and so on that they both can accurately model each other’s choices.”

“I could do that for my friends,” Pinkie Pie said. “No pony else.”

“You need something everypony—everyone—has in common,” Twilight said. “Say we found out there was intelligent life on Pluto, too far away to reach. What would we know about them?”

“That they like cupcakes?”

“Aside from that. The one thing that unifies all choice-making creatures is the logic of choice itself. We all optimize, and optimization has rules, is a set of rules. We all ask one question: how can I get as much value as possible?”

“Who do we ask?” Rainbow Dash said. Twilight shrugged.

“This idea is the idea of the total unity of all choice-making creatures,” Twilight said. “Like a perfectly managed garden, but this garden manages itself. If you want to ask Somepony about it, don’t look at me.”

“You’ve been explaining it this whole time.”

“Well….”

The blind journey up the mountain continued for several hours until the mountain seem to plateau. Twilight instructed the Breezies to clear the smoke away, which they didn’t. Then Fluttershy asked the Breezies to clear the smoke away, which they did while Twilight grumbled.

They were standing on a platform of rock surrounded on all sides by a vortex of smoke. Twilight wondered idly if somepony shouldn’t be trying to tempt them. It was impossible to see how high up they were, but judging by the length of the journey and how steep so much of it had been, it wasn’t exactly a hop, skip, and a jump down the cliffside to Appleloosa.

Across from them was the entrance to a cave. The Breezies cleared that of smoke as well, and they stood guard at the entrance to the cave while Twilight and the others ventured in. With the smoke outside, it was pitch-black in the cave.

Twilight lit a light from her horn. "We've been here before. Come on, girls."

They journeyed into the cave. Twice long jets of smoke came their way; each time Twilight split them with a two-sphere. The smoke gathered behind them while they went deeper into the cave.

Finally Niddhog was there, curled up on a bed of gold and jewels that was dazzling even to Twilight, who had grown up in Canterlot. Beside her she could hear Rarity drooling and a quiet “Meh” from Rainbow Dash.

Niddhog himself was an incredible beast, with scales that Twilight would have called rust-red if she hadn’t been told they changed and shined a dazzling array of colors in the light. Two proud horns protruded from his head, and ridges ran down his back, leading to a spiked tail that curved around to his face, a loop most awesome and ferocious. And all over him shadows danced as Twilight's light trembled.

Niddhog exhaled, but it was smokeless, mere impatience. Well, there was no point in wasting time. Twilight spoke.

“Hello, Niddhog. I am Twilight Sparkle, Chief Executive Economist of the Daughter of Ponyville, and these are my friends. We are here to slay you, unless you would accept payment to leave Equestria free of your smoke, and also we demand recompense for a cow, an apple orchard, and a library. I prepared the relevant accounts.” She levitated a short but dense book to Niddhog, who snorted a thin stream of flame that incinerated it. “Okay, then plan B. By order of Princess Celestia, your death is decreed.”

Niddhog opened his mouth, but rather than flame, he spoke. Shouted, rather, if the word were powerful enough, he screamed, raged; if sound were fire the earth itself might have melted; Niddhog’s roar was one meant to intimidate dinosaurs. Twilight only knew that the right side of her ribcage suddenly felt brittle, and as for the words themselves, they could only be understood after bouncing off the ground and walls a few times.

LEAVE ME BE, YOU LITTLE PONIES!

Twilight waited until she was sure she had stopped shaking. “No.” In the aftermath of Niddhog’s rage, it sounded so quiet, so meaningless, like a single star amidst the infinite darkness. “Not even for Equestria’s sake, although you must be stopped for Equestria’s sake. Not for the vengeance of my friends, although they deserve their revenge. For my sake, because you destroyed my library, is why I came here to defeat you. It couldn’t possibly be any other way. Know this, Niddhog, King of Dragons, Beast of Flame and Devourer of Books, you are never going to bring me down—“

Niddhog’s clawed hand shot forward faster than Twilight could blink, seized her by the horn, and squeezed.

All light vanished.

It was Piney Bravehoof who was the first to notice the blood trickling out from under Chief Thunderhooves’ tent, and it was Piney Bravehoof who stood closest to Little Strongheart as she opened the tent flap and examined the corpse. He noticed how sharp her eyes were, how bloody her mouth seemed. Then she looked at him, and he saw her intent to kill him, and he saw within himself that despite his name he lacked the courage to do anything about it.

“The pony Braeburn threatened no less last night.” Little Strongheart did not need to shout. Everypony in the camp heard her. “Ponies cannot be trusted. They keep no contract, they have no honor, and they respect nothing sacred. We attack.”

“At dawn?” a buffalo said.

“No. Now.” Her eyes found his. “Piney Bravehoof, who has suffered to be the one who found Chief Thunderhooves, will have the honor of leading us into battle. You will be the first to have revenge.”

Piney Bravehoof found that his voice didn’t work. He swallowed twice and nodded jerkily. “Yes, Chief Strongheart.”

Just between the surface of Niddhog’s claws and Twilight’s horn was a lavender glow. Slowly, surely, she forced his claws apart. Brilliant light filled the cave, pulsing like it was brimming with anticipation for something more precious than mere sight.

“—and you’re never going to break this part of me,” Twilight said. Niddhog snorted and withdrew his claws, rubbing them together. “My friends are here to make a sound. Not slaying just ‘cause externalities! We’re here to let you know that you have got to go. My math is magic, and it’s about to blow! MATRIX MODE: ACTIVATE!”

Twilight’s horn glowed lavender. It doubled on itself and then doubled again, Twilight’s eyes shut in fierce concentration. Then she opened them, and they were glowing white, and the magic released and passed over them in a wave that left them transformed in its wake.

Rainbow patterns, a dubstep beat, and the smell of apples met something floral; lightning crackled and little pink butterflies danced; a cake came out of nowhere and hit Niddhog in the snout. For a brief, embarrassing moment, the ponies were clothed in white tops and frighteningly short skirts that thankfully vanished soon after.

Golden chains appeared from each pony to each pony and latched tight, to what exactly was hard to say. Twilight flexed her magic, and felt the chains respond. She felt the power her friends were feeding her and what she gave them in return, not mere magic, but the truth of what it was, something that transcended pale words and linear numbers, the experience that Fluttershy thought could not be spoken could instead be transmitted through these chains.

They rose into the air, surrounded at all times by shivering light that was red, orange, blue, and others by turns.

“As individuals we may be weaker than you, Niddhog,” Twilight said, “but united as one you cannot defeat us! Our inputs are the full sum of our output, and we are the industry of antipollution! TRANSFORMATION: ECHELON SHIFT!”

The Markov chains pulled them about, changing position. Now the golden chains extended from Rainbow Dash and Applejack to Twilight, and from Twilight to Pinkie Pie to Rarity to Fluttershy, their eyes all glowing white. Twilight floated at the fore, brimming with magic.

“In Echelon Mode I have three powers: to shift in order all those who are connected by the Markov chain; to multiply any of their powers by a scalar of my choosing; and to add their multiplied powers to that of any other’s! You may have the power of externalities, o King of Dragons, but it is mere emptiness. We are filled with the limitless entirety of our own friendship, like the number pi, infinite and yet within our grasp! In the name of the sun, I’ll punish you!”

Niddhog roared and shot forward. Twilight’s horn glowed. She sent the scaled-up Applejack to meet the scaly dragon.

“ECHELON BUCK!”

Piney Bravehoof’s legs were turning to corn mash.

Say it.

He was at the fore of the stampede, and the stampede was pointed at Appleloosa.

Say it.

The smoke made everything so dark. The top of Hark Mountain was entirely obscured. Piney Bravehoof found he couldn’t open his mouth.

Everypony knows!

Piney Bravehoof’s knees locked up. But he still continued to run.

He knew that everypony knew that Little Strongheart had killed Chief Thunderhooves. He even thought that everypony knew that everypony knew that Little Strongheart had killed Chief Thunderhooves.

But he didn’t know if everypony knew that.

No pony said anything. They continued to run toward Appleloosa.

The force blasted the dragon through the back of the cave and out. He took to his wings, stretched to his full size and height, blowing the smoke around him away, a mighty beast who had once fought tyrannosaurs and made the brontosaurus mythical.

The six perfectly united ponies pursued.

Niddhog reared back, stretched his neck to the heavens, and when his head came down it was spewing fire hot enough to melt the mountain rock.

“ECHELON CLOPEN SET![1] ECHELON VECTOR BEAM!”

A shimmering translucent shield formed around the ponies. The fire splayed out harmlessly against it. At the same time, a huge purple laser blasted through the shield and clipped Niddhog, sending him spinning.

[1]Mathematics, being the purest form of logic, naturally has room for sets that are simultaneously open and closed. Look, I can’t make this stuff up.

He righted himself, snarling; the ponies were coming; he extended a claw to smash them. The ponies stopped, rotated around the stationary point that was the tip of his claw; they kicked off his arm, denting it, and another laser beam, blocked just in time, ripped the scales off the back of his hand.

Niddhog whipped his tail, scattering the ponies, but the golden chains that bound them were untouched. This time, instead of fire, he spewed smoke.

“ECHELON PARTY CANNON!”

Balloons the size of zeppelins appeared in the air, enclosing the smoke, including the stream Niddhog sent at them. As the balloons floated away, they took the smoke with them. The noxious gas began to clear, and sunlight streamed in.

Twilight gathered the ponies together in formation again, using, though she didn't know it, the ancient fighting style of the Hessian warriors. Niddhog began to fly, and so did they, matching his every turn with a rainbow explosion. He spat fire that was dodged or blocked, and it was only by every trick and reflex he had honed over millions of years that he was able to avoid a killing blow from the laser beams that ripped the scales off his body. As they scattered in the air, glinting different colors in the light, they were wrapped in a blue glow and called to Rarity, who seemed to be gathering them. Twilight sent her power.

“ECHELON QUEUE DU SINGE!”

An enormous blue fabric erupted out of Rarity’s horn; it wrapped around Niddhog’s tail. With a lurch and bugging eyes, he was yanked backwards.

“ECHELON FLANK SPANK!”

Rainbow Dash’s small but incredibly powerful kick exploded in a rather embarrassing spot. Niddhog howled and whipped around, tearing apart the fabric. His mouth opened to unleash fire, and then he saw the kindest face and heard the loudest whisper ever.

“UM VERY SORRY IT’S NOTHING PERSONAL ECHELON CHECK YOUR PRIVILEGE!”

Niddhog’s mouth hung open; no fire did he spew. His gaze was drawn to that impossibly cute pony, her demure voice and the way her hair fell over her face, and then a lavender flash and a purple Unicorn obscured his view of her.

Little Strongheart’s magic blasted a tunnel through the smoke. There were only the usual defenses set up, as if the Appleloosans were playing the part of innocent ponies. But there was no innocence in the interactions between ponies and buffaloes, not even the absence of those interactions. For there was everything that had come before, and everything that would come after, and compared to that the here-and-now was an infinitesimal fraction of nothing. All those who had no ears to hear would be made to look.

Soon after Piney Bravehoof was dead, the victim of a hard-baked and well-thrown apple pie. All the more reason to fight.

Little Strongheart tore down the lamps. She shook the foundations of the buildings. And she found Braeburn amidst the chaos, an apple pie in each forehoof. She made them burst; she made the ground shake underneath his hoofs; she made him run.

Little Strongheart pursued him through the dusty streets, tearing down the lampposts as she went. Behind her a battle was raging; in front of her was a delicious prize.

She might have stopped to notice that it wasn’t getting any darker in Appleloosa no matter how many lamps she tore down. But who, in that moment, could have made her listen?

The ground swelled around Braeburn. It took hold of his hoofs, wrenching them apart, splaying him out on the ground. Little Strongheart walked up to him. She put her teeth on his right ear and closed her jaw until her teeth touched.

She was about to kill him when the heavens opened up. Light streamed down, and a voice spoke.

Twilight Sparkle chose her position in the air. Gravity called to her. She ignored it.

“Niddhog, your defeat was inevitable.” she declared. “Because economics is science. And math is magic! Your end is at hoof. FORBIDDEN TECHNIQUE: INFINITE SCALAR! ECHELON MATRIX BEAM!”

A laser, predominantly purple but flecked with yellow, orange, blue and pink ripped through the air, leaving scattered and torn molecules in its wake. It struck Niddhog in the belly and scythed up through his chest. Like an overcooked pie, he split open.

Nopony below heard the great battle raging above. But when the smoke cleared away, and pony and buffalo alike saw that just overhead a dragon and a matrix of ponies like were doing battle like Satan versus a host of angels at the End of Days, their own grievances seemed suddenly meaningless in the face of the real enemy and true power.

Little Strongheart looked up. Fiery wisps like the sun’s pent-up rays fell down from heaven. So did enormous drops of rainbow blood like a rainbow that got wet and started to run, like a promise that had lasted a very long time and was now beginning to fade.

Some of the rainbow blood landed on the red crystal pinned to Little Strongheart’s chest.

The dragon’s long torso was pulled out and open as if by a deranged taxidermist. His sightless eyes were pointed up. Smoke escaped his open mouth and was taken away by the wind. Then, after a long, silent moment, he began to fall.

A voice louder than any Little Strongheart had ever heard screamed down at her from the heavens.

“LITTLE STRONGHEART! HEAL NIDDHOG!” Next Chapter: Going Up a Level in Friendship Estimated time remaining: 10 Minutes