The first ember-lizard hit the ground still trying to bite.
Its body was no larger than a forearm, all cracked black shell and ember-red eyes, but the hiss coming from its throat sounded far too hot for something so small. Sparks spat from between its teeth each time it snapped at the air.
Pei Zhen backed another step toward the bend in the trench, one hand clamped over his bitten forearm. "What did you open?"
Gu Yan did not answer.
Because the wall was answering on its own.
The gap split wider by slow degrees, brick grinding against older stone somewhere beneath it. Dry heat pushed out in thick breaths, no longer the tired warmth of buried channels but something meaner—heat trapped too long and angered by light.
Another ember-lizard leapt from the opening.
This one came at Gu Yan's face.
He twisted aside and let it scrape across his sleeve instead of his throat. The creature hit the trench wall, rebounded, and launched again with a shrill crackling hiss. Gu Yan struck with the back of his wrist. It was like hitting a fired shard wrapped in sinew. The blow knocked it off line but did not break it.
Pei Zhen had better luck. Or worse temper.
He smashed the first creature under his heel until the shell split and the glow inside spilled out like burning sand. But before he could breathe, two more red-eyed shapes crawled from the widening gap and dropped to the trench floor.
"Back," Gu Yan said.
Pei Zhen shot him a look. "You give orders now?"
"Only the useful kind."
The third creature lunged low. Pei Zhen kicked it away, but the fourth went for his leg at the same moment. Gu Yan moved before he thought. He drove his heel down, pinning the thing's tail, then brought the fragment up with his free hand.
The reaction was instant.
The crooked lines on the blackened metal flashed dull red. The ember-lizard shrieked—an ugly, metallic sound—and twisted so violently its cracked shell burst along one side. Heat rushed out of it, not into the air, but into the fragment.
Gu Yan felt that heat shoot up his arm.
Too fast.
His chest tightened. The old weak line under the ribs flared as if something had driven a hot nail beneath the bone.
He nearly lost his grip.
That was the old fire's temper.
Not gentle. Not patient. It only knew how to take heat, hold heat, and force heat through whatever touched it.
Gu Yan exhaled sharply and dropped his shoulders instead of tensing them. Back first. Then frame. Then force. The habit he had felt in the fragment the night before returned like a hard hand on the spine.
The pain narrowed.
Not gone. Narrowed.
He drove the fragment down again.
The pinned creature cracked open completely this time and collapsed into a scatter of black shell and ember dust.
Another hiss came from the passage.
Then another.
Gu Yan looked into the gap and felt his scalp tighten.
At least six more ember-red eyes gleamed in the dark beyond.
Too many to fight one by one in a trench this narrow.
Pei Zhen clearly saw the same thing. Whatever he had intended before—taking the fragment, reporting later, turning the whole discovery into leverage—those plans had just been shoved behind a simpler truth.
If they stayed here, they would be eaten down to bone by furnace vermin.
"There," Gu Yan said, pointing not at the opening but at the half-broken wall brace to the right.
Pei Zhen understood at once.
Together they slammed into it.
Old brick groaned. Ash dropped. One of the loosened support stones cracked free and fell across part of the gap just as three more ember-lizards sprang out. Two struck the fallen stone and bounced back inside. The third squeezed through, hit the trench floor, and charged.
Pei Zhen met it with the broken grate he had snatched from the ground. The iron rang once. The creature stuck to it like a coal clinging to tongs, shrieking sparks into his face.
Gu Yan seized a broken brick and smashed the thing sideways. It hit the wall and split.
The temporary block would not hold long.
Already the stone was shifting under pressure from the other side.
Pei Zhen threw down the grate. "We run."
"Yes," Gu Yan said, "deeper."
Pei Zhen stared at him. "Are you mad?"
"Toward the old heat, they'll follow slower. Toward the yard, they'll follow faster."
That was guesswork.
But it was good guesswork.
The creatures had come out in a burst the moment the wall first opened, like scavengers reacting to a break in containment. The open trench leading back to the outer court was the worst place to test the answer.
The block shifted again.
This time Pei Zhen did not argue.
They turned and ran.
The trench narrowed, then split. Gu Yan felt the fragment pull hard to the left, toward a side passage half choked with old ash and fallen slats. He took it without hesitation. Pei Zhen followed with a curse.
The space beyond was lower, hotter, and older.
The brickwork changed first. The sect's present kiln channels used flatter, more regular stone. These were thicker, darker, and carved with fine furnace grooves that carried old heat in crooked lines through the wall.
The fragment in Gu Yan's hand throbbed once like a second pulse.
Behind them came the skittering sound of claws on brick.
Not far enough.
Gu Yan dropped to one knee at the side passage's mouth and pressed the fragment against one of the groove-lines running through the wall. He did not know exactly what he was doing. He only knew that the heat in this place belonged to the same family as the thing in his hand.
For one dangerous beat, nothing happened.
Then the groove answered.
Dull red light spilled through the wall pattern. Old ash in the air jumped as hot breath pushed through hidden vents. From deeper in the side passage came a grinding click, then another.
The floor shifted.
A panel of old brick dropped half a hand's width at the entrance.
The first ember-lizard hit it headfirst and shrieked. The second clawed over the first, but before it could squeeze through, a crossflow of furnace breath blasted from the side wall and swept both creatures back into the trench like leaves in a firestorm.
Pei Zhen actually laughed once, breathless and disbelieving. "You really opened something."
"Help me hold it."
Because the panel had not locked.
It trembled under the pressure of small bodies striking from the other side.
Pei Zhen slammed both palms against the dropping stone. Gu Yan did the same, fragment still burning in his right hand. The pressure was worse than weight. Weight was honest. This was jolting, repeated, irregular. Every hit sent small violent shocks through his arms and into his ribs.
His body wanted to answer with force.
That would have failed.
Instead, he settled lower, let the pressure travel through his back and hips, and used the heat in the wall to read the rhythm of the impacts.
One strike. Pause. Two fast. Scrape. Leap.
Again.
Again.
Again.
For the first time, the old fire did not merely hurt him.
It taught timing inside pain.
After what felt like an entire watch, the impacts slowed.
Then stopped.
Only then did Gu Yan pull his hands back.
The stone panel remained lowered, still hot, still faintly red along the old grooves.
Pei Zhen leaned against the wall and looked down at his bitten forearm. The skin around the wound had darkened, as if fine soot had settled beneath it.
"That thing burned me from the inside," he said.
Gu Yan spared it one glance. "You're still standing."
"For now."
The side passage lay quiet around them, quiet enough that both men could finally think.
Which was dangerous in another way.
Pei Zhen straightened first. "You know I can't pretend this didn't happen."
Gu Yan looked at him. "I know."
"And if I report it—"
"You'll have to explain why you were following me in a forbidden trench in the middle of the night."
Pei Zhen's mouth tightened.
For a few breaths they stood there measuring each other while the hidden passage breathed old heat around them.
Then Pei Zhen's eyes shifted past Gu Yan's shoulder.
His expression changed.
Not calculation this time.
Surprise.
Gu Yan turned.
Ten paces deeper into the passage, beyond a drift of old ash and two fallen support beams, stood a narrow furnace alcove set into the wall. Its front was cracked. Its inner chamber was dark. But above it, carved into the stone, was a mark so weathered it should have meant nothing.
The fragment in Gu Yan's hand burned harder the moment he saw it.
It was not the sect's present furnace sigil.
It was older.
Rougher.
And almost the same as the line that had flashed across the fragment's surface the night before.
Pei Zhen spoke first, voice low now without him meaning it to be. "That wasn't built by the current sect."
"No," Gu Yan said.
For the first time since following him, Pei Zhen sounded less like a man chasing advantage and more like a disciple standing in front of something he did not understand.
"What is this place?"
Gu Yan looked at the old alcove, at the cracked furnace mouth, at the grooves that still carried hidden heat after who knew how many years.
He thought of the image the fragment had shown him in the dark. A split furnace mouth. Old stone. Heat moving through hidden channels.
And then, from somewhere inside the cracked alcove, came a single low knock.
Not from settling brick.
Not from a beast scratching.
A knock.
Both men went still.
The fragment in Gu Yan's hand pulsed once more, hotter than before.
Whatever lay deeper in the hidden passage had not finished answering.
And this time, it sounded like something on the other side had heard them too.