Saturday before dawn, Rowan put his hand on Lawrence's shoulder. Lawrence was dressed in under a minute. They found Iris already waiting in the common room, boots on, knapsack over one shoulder. She'd borrowed a pair of Lawrence's work gloves, which were too big for her and made her look like she was about to clean out a gutter.
"You've got a week left on the potion," Iris said, looking at Rowan's left hand.
"It's fine in the cold if I keep it moving."
"It's October in the Scottish Highlands."
"I cast a warming charm on the glove. It'll hold for a few hours."
She looked at Lawrence, who was already at the door. He'd wanted to go the first night back. Rowan had talked him into waiting for the weekend, and that was as much waiting as Lawrence had in him. His mother's hands shook worse every month. The moonstone wasn't going to get closer to forming while they sat in the common room.
Iris pulled on her gloves. "If your hand locks up, we turn back."
"Agreed."
They left through the passage behind the third-floor tapestry and crossed the grounds in the dark. The lake was black. The Forest was a line of darker black at the southern edge. Nobody talked. Lawrence had been like this since August, and Rowan had stopped trying to fill the silences because Lawrence didn't want them filled.
Iris checked the bearing with a Point-Me and corrected Rowan's map three times in the first twenty minutes. She had a better eye for distances than he did. The first correction was a stream he'd placed fifty yards too far west; the second was a rock formation he'd sketched from memory and gotten wrong; the third was an entire section of trail he'd invented by combining two separate paths into one.
"Your mapping is terrible," she told him.
"I know."
"I'm doing the map from now on."
The ground rose after the first hour. The trees changed from broadleaf to pine, the soil going rocky underfoot. The mist thickened at the higher elevation and cut their visibility down. Both of them had their wands out.
A branch snapped off to the left.
Rowan stopped. Iris was beside him half a step later, wand left while his covered right. Lawrence dropped into a crouch behind them.
The centaur came around a pine trunk. Young by centaur standards, chestnut coat, longbow unstrung across his shoulders. He looked at the three wands pointed at him and waited.
"Starweaver sent word," he said. "You sought moonstone."
"Still am," Rowan said. "Iris Caldwell and Lawrence Goode."
"Thornmane. Starweaver is my mother's brother." He looked up the slope toward the thinning trees. "You're heading into the highlands."
"What's between here and the ridge?"
"Thornbacks along the treeline. Dormant during the day if you don't disturb them." He turned his head slightly, glancing up through the canopy. "Their numbers have been growing. They've pushed close to our grazing lands and we can't drive them back permanently. Starweaver said you accepted a debt when you took what we offered. If you thin their numbers in the high passes, we'd consider it a step toward repayment."
"How much of the debt?"
"I can't answer that yet." He looked at Rowan directly. "Be careful past the first ridge. There are old structures in the high valleys. We don't go near them."
"Why?"
He was already walking away.
Iris watched him go. "Did he just tell us to fight the thornbacks and then refuse to say why we shouldn't go past the ridge?"
"He told us what he wanted us to know," Lawrence said. "The rest was a warning."
They climbed for another hour. The forest thinned and the terrain opened up, the pines growing sparse and stunted and the ground turning to heather and exposed rock. The wind picked up as soon as they cleared the tree line. Rowan's warming charm guttered and died inside five minutes, the wind shearing it away faster than he could recast, and his left hand went numb shortly after. He switched his wand to his right.
Iris kept mapping. She'd worked out a system, parchment braced against a flat stone, ink charmed to resist the wind, compass headings noted at each change of slope. She was good at it. She caught features Rowan had missed entirely: a spring that emerged between two boulders, a line of iron-stained rock that might indicate a fault, a series of shallow caves cut into the southern face of the ridge that looked natural but were too evenly spaced.
"Those aren't natural," Iris said, looking at the caves.
"No." Rowan studied them. The spacing was regular. Someone had carved them, or enlarged them, a long time ago. "Mark them. We'll come back."
Lawrence worked the ground ahead of them. He turned over rocks and checked exposed mineral faces, splitting likely stones with a tap of his wand and examining the interior. His hands were raw from the cold. He didn't stop.
They found their first promising reading on a south-facing slope at the top of the second ridge. Rowan's detection charm picked up elevated magical resonance in the feldspar, and for ten minutes all three of them searched the outcrop, turning every stone, checking every crack and hollow.
Nothing. The resonance was there but the moonstone wasn't. The conditions were right but the centuries of accumulation hadn't finished the job.
"Could be a hundred years away from forming," Rowan said.
Lawrence straightened up and looked at the next ridge. "Higher."
They climbed. The terrain got worse. The heather gave out and there was nothing but rock and scree and patches of old snow in the shadows of boulders. Rowan's boots slipped on loose stone and he went down hard on one knee, tearing his trousers and scraping his palms. Iris caught his arm before he slid further.
"All right?"
"Fine."
He wasn't fine. His left hand was a block of ice and his knee was bleeding and they were further from the castle than any student was supposed to be and the wind was coming in gusts now that made conversation difficult. But the second promising reading came an hour later, stronger than the first, on a high exposed shelf where the lunar sightlines were unobstructed in three directions.
They searched for forty minutes. Lawrence was methodical and thorough and his face got tighter with every stone he split open and found empty. Iris mapped the shelf's orientation and calculated the moonlight angles for each phase. The numbers were good. The conditions were as close to ideal as they were going to find. And there was nothing there.
Lawrence sat down on a rock. He pressed his palms flat on his knees and stared at the ground between his boots.
Rowan gave him a minute. Iris was still mapping, adding detail to the parchment, working outward from the shelf toward the next ridge. She looked up and caught Rowan's eye and shook her head slightly. Not now.
"The readings get stronger higher up," she said eventually, rolling the parchment.
Rowan looked at the sun. Past its peak. They'd been out since before dawn. "We should head back."
"One more ridge," Lawrence said.
"We don't have the daylight."
"We have three hours."
"Three hours to get back. We barely have that."
Lawrence looked at him. His jaw was set and his eyes were flat and patient in a way that had nothing to do with calm.
"Next Saturday," Rowan said.
Lawrence stood up.
They started back down. The descent was harder than the climb in places where the scree had shifted. Iris went first, picking the route, and Rowan followed and Lawrence came last. Nobody talked. The wind dropped as they got below the ridgeline and into the shelter of the terrain.
They were back among the pines, an hour from the castle, when Iris stopped.
"Rowan."
He stopped. She was looking at the ground.
Silk. A strand of it, strung between two tree roots, thick as twine and sticky when Rowan touched it with his wand tip. It stretched and held. Thicker than spider silk, tougher, with a faint iridescent sheen in the failing light.
Another strand, a yard to the left. Then another. The ground between the pine roots was laced with it, an irregular mesh that radiated out from beneath a slab of mossy rock.
"That's a thornback web," Iris whispered. "Poppy showed me a diagram in her field guide. The radial pattern, they lay it flat, across the ground."
Rowan looked up. The canopy was thickening overhead and the light was going and nothing moved in the mist between the trunks. The web was fresh, the strands taut and clean.
"Thornmane said they den along the treeline," Lawrence said.
"We're at the treeline."
A sound. Not a branch snapping. Something clicking. A rapid, dry staccato, like someone running their fingernail across the teeth of a comb, except it came from multiple directions.
Iris took a step backward. Her boot caught a strand of the web. It held for a moment, stretched, and snapped with a sound like a plucked string.
The clicking stopped. All of it, at once, in every direction.
Then something moved in the undergrowth to their right. Low, fast, and far too large for any spider Rowan had seen in two years of living near the Forbidden Forest. A dark shape, bristling with spines along its back and legs, slipping between the tree roots with a horrible fluid efficiency. He saw the sheen of its exoskeleton before the mist swallowed it again.
Another one. To the left. This one didn't hide. It came around the base of a pine and stopped in the open, ten feet away, and Rowan got his first clear look at a thornback.
It was the size of a large dog. Eight legs, each one studded with barbed spines from joint to claw. The body was armoured in overlapping plates that looked like dark horn, ridged and scarred. Too many eyes, clustered in a raised dome at the front of its head, reflecting the dim light in wet black points. Mandibles the length of Rowan's fingers worked slowly, testing the air.
More clicking. Behind them now. And above. Rowan looked up and saw another one clinging to a pine trunk fifteen feet overhead, perfectly still, its belly pale and exposed against the bark.
"That's an ambusher," Iris breathed. "The ones in the trees shoot webs."
Rowan counted. Three on the ground that he could see. Two in the trees. The clicking said there were more. The web under their feet meant they were standing in a hunting ground, and the snapped strand meant every thornback in range knew it.
Lawrence had his wand up. His hand was steady. "Fire," he said. "They're vulnerable to fire."
"If we start casting, the rest of the den comes," Rowan said.
The thornback on the ground took a step closer. Its spined legs made no sound on the pine needles. The mandibles opened wider and Rowan could see the venom glistening on the curved tips.
Iris pressed her back against his. Lawrence shifted to close the triangle. Three wands out, three directions covered. The mist crept between the trees. The clicking started again, louder, from everywhere.