If you want to build a machine that replaces a human leg, you can't just draw it on paper. You have to bleed for it. You have to sweat until the salt stings your eyes and your hands are blistered raw.
For the next two weeks, Toren's workshop became our world. It wasn't a classroom anymore; it was a prison of coal dust, hot iron, and burning oil.
It was the hardest thing we had ever done.
"Strike!" I yelled, my voice cracking over the roar of the forced-air fire.
Kaela swung the heavy sledgehammer. She wasn't wearing her leather armor; she was stripped down to her wool undertunic, sleeves rolled up to her shoulders, soot smeared across her face like war paint.
CLANG.
The hammer hit the glowing bar of steel on the anvil. Sparks sprayed like angry fireflies, bouncing off my leather apron.
"Again!" I ordered, turning the tongs. "The metal is cooling. Hit it while it's soft! Don't hit it like an enemy; hit it like a drum. Rhythm, Kaela!"
CLANG.
She grunted, swinging from her hips. Her arms were trembling. We had been at this for four hours straight. The steel we were shaping was stubborn. It was old plow-iron—tough, dense, and folded a dozen times. Toren had saved it for years, waiting for a special project. Now, we were beating it into a shin bone.
"Lysara, heat!" I shouted without looking.
Lysara was manning the bellows. It wasn't the glorious, magical work she was used to. It was manual labor. It was a rhythm game. Pump, hold, pump. If she went too fast, the fire got too hot and burned the carbon out of the steel, making it brittle as glass. If she went too slow, the metal cooled and cracked under Kaela's hammer.
"Adjusting flow," Lysara muttered through gritted teeth. She leaned her slight weight onto the long wooden lever. The coals roared, shifting from a dull orange to a blinding white-yellow. Sweat dripped from the tip of her nose, sizzling on the hot bricks of the forge.
I watched the metal.
Snap. Infinite Skill engaged.
I didn't just see the color. I saw the structure. I saw the grains of the iron aligning under the heat and the impact. I saw the stress points fading as Kaela hammered them out, forcing the metal to accept its new shape.
"Harder," I said. "Flatten the curve. It needs to be a shell, not a bar."
Kaela roared—a frustration-fueled sound—and brought the hammer down with everything she had.
CLANG.
The metal flattened perfectly.
"Hold!" I signaled. "Quench it."
I grabbed the tongs with both hands. The Hollow in my chest hummed, feeding a trickle of strength to my arms so I could lift the heavy piece without shaking. I plunged the glowing steel into the barrel of oil.
HISSSSSSS.
A cloud of white smoke erupted, smelling of burnt grease. The oil bubbled violently.
I held it there, counting the seconds. One. Two. Three.
I pulled it out. The steel was black, ugly, and steaming. But it was the shape of a shin guard. The outer casing for the piston.
I dropped it on the workbench. My arms felt like jelly.
"Break," I wheezed.
Kaela dropped the hammer. It hit the dirt floor with a dull thud. She collapsed onto a stool, wiping her face with a dirty rag.
"I hate blacksmithing," she announced, reaching for a water skin. "It's just fighting a rock that doesn't fight back."
"It fights back," Toren said from the doorway. He was leaning against the frame, watching us with a critical eye. "It fights with resistance. If you don't respect the iron, it breaks your tools."
"Or your foot," I added, looking at the heavy anvil.
I walked over to the other bench. The "Clean" bench.
This was where the fear lived.
Laid out on a white cloth were the pieces of the inner mechanism. The brass cylinder I had polished by hand for three days until I could see my own tired face in the reflection. The steel rod that acted as the piston. And the seals.
The seals were the problem.
We had tried to assemble the prototype three days ago. We filled the tube with lamp oil, inserted the piston, and applied pressure.
It had failed.
When I pushed the rod down, oil had sprayed out of the top, coating the bench in slick, useless fluid. The leather washers I had cut weren't tight enough. The pressure blew right past them.
Kian had been watching that day. He hadn't said anything. He had just looked at the puddle of oil, then at his stump, and then he had hobbled out of the room.
I couldn't fail him again.
"We can't use rubber," I muttered to myself, picking up a failed washer. It was dry and cracked. "We don't have rubber trees in the valley."
"What about cork?" Lysara asked, joining me at the bench. She was massaging her shoulder, which was stiff from the bellows. "Alchemists use cork for high-pressure vials."
"Too brittle," I said, tossing the leather ring down. "Cork crumbles under impact. This leg is going to be slamming into the ground a thousand times a day. We need something flexible but tough. Something that swells when it gets wet but doesn't dissolve."
I stared at the leather. It was just cured cow skin. It was porous. Oil leaked through the pores.
"It needs to be saturated," I realized. "It needs to be part liquid already."
I looked at Toren.
"Dad," I said. "The tallow. The heavy stuff you use for the wagon axles."
Toren raised an eyebrow, pushing off the doorframe. "That's thick grease, Ren. It's like glue. It's meant for wheels, not... whatever this is."
"Exactly," I said, my brain racing. "If we boil the leather in the tallow... not just dip it, but boil it... the grease will replace the water in the hide. It will make the leather swell. It will be self-lubricating."
Toren walked over. He picked up a strip of leather. He nodded slowly.
"It might work," he said. "The heat will open the pores, and the fat will seal them shut when it cools. But it'll stink."
"I don't care about the smell," I said. "I care about the seal."
I looked at my team. They were exhausted. Kaela was nursing a blister on her palm. Lysara looked ready to pass out.
"One more hour," I said. "We boil the seals tonight. We assemble tomorrow."
Kaela groaned, throwing her head back. "Ren, come on. My arms are going to fall off."
"Do you want Kian to walk or crawl?" I asked quietly.
Kaela stopped complaining. She stood up. She grabbed the water skin and splashed some on her face.
"Fine," she said. "But if this leg explodes, I'm hitting you with the hammer."
"Deal," I said.
The next morning, the smell of boiling tallow hung over the workshop like a heavy fog. It smelled like burning fat and old boots. It was disgusting.
But the seals were ready.
I pulled the leather washers out of the pot with tongs. They were dark, swollen, and slick with grease. They looked disgusting. They looked perfect.
"Okay," I said. "Assembly time."
We gathered around the clean bench. Kian was there, sitting on a stool, leaning on the crutch I had carved for him. He was watching us with an intensity that made my skin itch. He hadn't said much in the last few days. He just watched, his eyes dark, waiting to see if we were selling him a lie.
I picked up the piston rod. I slid the greasy washers onto the end, locking them in place with a brass nut.
"Tight fit," I muttered.
I picked up the brass cylinder. I coated the inside with a thin layer of fresh oil.
I pushed the piston in.
It didn't slide easily. I had to push.
Schluuuuck.
It made a wet, sucking sound. A perfect seal.
"No air," I said. "Only oil."
I poured the refined lamp oil into the chamber. I screwed the cap on. It was a heavy brass cap with the copper rune etched into the top—the interface.
"The knee," I said.
Kaela handed me the steel ball-joint we had forged. I snapped it into the socket at the top of the shin guard. It moved smoothly, greased with the same tallow.
I attached the piston to the frame. I bolted the foot plate—a piece of steel with a tread cut from an old cart tire—to the bottom of the rod.
It was done.
It lay on the table, gleaming in the morning light. It looked brutal. A mix of armor and machine. Heavy steel, polished brass, and dark leather. It didn't look like a human leg. It looked like something a golem would wear to kick down a castle gate.
"Mark I," I said, wiping my greasy hands on my apron. "The Piston Leg."
"It looks... angry," Lysara whispered.
"It is angry," I said. "It's built to kick things."
I looked at Kian. "Ready?"
Kian swallowed hard. He looked at the metal limb. Then he looked at his empty pant leg.
"Yeah," he whispered.
I picked up the leg. It weighed about eight pounds. Heavy, but balanced.
I knelt in front of Kian.
"This is going to feel weird," I warned. "It's cold."
I slid the upper cuff—padded with wool—onto his thigh. I tightened the leather straps. Not too tight to cut off circulation, but tight enough so it wouldn't slip.
"Does it pinch?" I asked.
"No," Kian said. He was staring at the metal foot resting on the floor. His hands were gripping the edge of the stool so hard his knuckles were white.
"Okay," I said. "The interface."
I took a small copper wire connected to the rune on the piston. I pressed the end of the wire against Kian's skin, right above the knee, where the nerves were still active. I taped it down with a strip of linen.
"This is the trigger," I said. "It connects to your mana field. To the Void-Touch."
Kian flinched. "I don't know how to use that. I only know how to break things."
"The Void breaks things by pulling," I explained, looking him in the eye. "To push the piston, you have to feed it. You have to push."
I stood up and stepped back.
"Stand up," I said.
Kian looked at me like I was crazy. "I can't."
"You have two legs now," I said. "Stand up."
Kian gripped the workbench. He put his good foot on the floor. He hesitated. Then, he lowered the metal foot.
Clank.
Steel hit wood.
He put weight on it.
The piston compressed slightly—maybe half an inch—absorbing the shock. The oil pressure held. It didn't collapse. It didn't leak.
Kian's eyes widened. "It's holding."
"Trust it," I said.
He pushed off the bench. He stood.
He wobbled, arms flailing for balance. Kaela stepped forward to catch him, but I held up a hand. "Let him find it."
Kian swayed. The metal leg held firm. He was standing on his own two feet for the first time in a week.
"I'm standing," he breathed. A smile broke across his face—fragile, terrified, but real. "Ren, I'm standing."
"Now walk," I said. "This is the hard part. When you step, you have to push a little bit of that cold energy into the rune. If you push too hard, the piston will fire and launch you. If you don't push enough, it's just dead weight."
Kian nodded. He focused. I saw the sweat bead on his forehead. I saw the grey shadows curl around his fingers as he reached for the Void inside him.
He took a step with his good leg. Then he tried to bring the metal leg forward.
He pushed the energy.
HISSS-THUNK.
The piston fired violently. The leg shot forward, slamming the foot into the floor hard enough to shake the tools on the bench.
Kian lost his balance. He fell backward, hitting the dirt floor hard.
"Kian!" Kaela rushed forward.
"I'm okay!" Kian shouted. He sat up, rubbing his elbow. He looked at the leg. He wasn't crying. He was laughing.
"Did you see that?" Kian gasped. "It kicked! It kicked like a mule!"
"Too much juice," I said, grinning despite myself. "You kicked the door down. Just... open the door."
He scrambled back up. "Again."
"It's hard," Kian gasped. "The energy... it wants to rush out."
"Control," I said. "Like a faucet. Drip, don't pour."
Kian gritted his teeth. He tried again.
Hiss-clack.
Better. A jerky step, but a step.
He took another. Hiss-clack.
And another. Hiss-clack.
He was walking. It was robotic, loud, and ungraceful, but he was moving across the workshop floor under his own power.
"It feels..." Kian paused, searching for the word. "Alive. It hums when I push it."
"That's the pressure," I said. "That's the power."
Toren stepped into the room. He had been watching from the forge. He walked over to Kian. He knelt and checked the welds on the knee joint while Kian stood there. He ran his thumb over the tallow seal on the piston. No oil on his finger.
"Solid," Toren said. "Good welds. The seal is holding."
He stood up and looked at me. There was a look in his eyes I hadn't seen before. Not just pride. Respect.
"You built a limb out of scrap metal and grease," Toren said. "The Guild Artificers would charge a thousand gold suns for a prosthetic half this good."
"The Guilds can eat dirt," I said, crossing my arms over my chest. The Hollow felt quiet for the first time in days. It wasn't hungry. It was satisfied. I had built something.
I looked at Kian. He was walking back and forth now, getting the rhythm. Hiss-clack. Hiss-clack. He was getting faster.
"It's not finished," I said to the room. "This is the Mark I. It's rough. It's loud. You can't sneak in that thing."
I walked over to the workbench and picked up my hammer. I weighed it in my hand. It felt good. It felt like a tool, not a weapon.
"We have a lot of work to do," I said. "Kian needs to learn to fight with that thing. He needs to learn to trust it so he doesn't hesitate."
I looked at Kaela. "You need to learn how to cover him. He's got a blind spot on the left now."
I looked at Lysara. "And you need to figure out how to shield the noise. If we're going to move silently, we need a dampening field."
"The acoustics of the brass are problematic," Lysara noted, pulling out her slate. "But I can calculate a counter-frequency."
"Good," I said.
The Captain had said they would be back. He had given me the Iron Coin, but he had also promised to collect the debt.
When they came back, we weren't going to be children hiding behind a smith. We were going to be a unit.
I tossed the hammer in the air and caught it by the handle.
"Class dismissed," I said. "Tomorrow, we hit the training grounds. Kian, bring your leg. You're going to learn how to kick through a door."
Kian stopped walking. He looked at me. The shadows in his eyes swirled, but they didn't look like a curse anymore. They looked like fuel.
He grinned. It was a sharp, dangerous grin.
"Yes, sir," he said.
We were broken. We were scarred. We were wanted by the law.
But we were standing. And we were armed.