The nearest thornback moved first.

  It came low and fast across the web, legs folding and extending in a blur, mandibles wide. Rowan hit it with a Flipendo that caught it mid-stride and threw it sideways into a pine trunk. The exoskeleton cracked on impact. It hit the ground, scrabbled, and got up.

  "So much for not casting," Iris said, and threw Incendio at the one flanking left.

  The fire caught. The thornback's armour plates blackened and it shrieked, a thin, piercing sound that cut through the clicking like a knife, and every other thornback in the treeline answered. The clicking became a roar. The undergrowth came alive.

  Lawrence cast a wide-arc Incendio that swept across the ground web. The silk caught and burned, fast and bright, throwing orange light up into the canopy. Three thornbacks on the ground recoiled from the fire line. Two more came through it.

  Rowan killed the one he'd knocked into the tree. Confringo. The detonation blew the armour apart and sprayed dark fluid across the pine needles. The second one from the fire line was on him before the smoke cleared, faster than the first, claws raking across his shield. He felt the impact through his wand arm and the shield buckled and he had to drop it and roll.

  Something hit the ground beside his head. A gob of silk, thick and steaming, that splattered across the dirt and hardened on contact. The ambusher in the tree had fired. He looked up and it was already reloading, its abdomen pulsing, another shot forming.

  "Iris!"

  She was already casting. Her Incendio hit the ambusher's pale belly and it let go of the trunk and fell, burning, trailing silk behind it. It hit the ground and curled and its legs pulled in tight and it stopped moving.

  The second ambusher fired from a different tree. The web hit Lawrence across the left shoulder and arm and pinned him to the pine behind him. He wrenched against it. The silk held. His wand arm was free but his torso was stuck and his feet were off the ground, the web's adhesion taking his full weight.

  Rowan put a Confringo into the tree trunk six feet below the ambusher. The detonation tore a chunk of bark loose and the shock knocked the creature free. It fell twenty feet and landed on its back and its legs churned the air. Lawrence hit it with Incendio from where he hung. It burned.

  "Get me down."

  Rowan tried to cut the silk with Diffindo. The first cast scored it. The second cut halfway through. The third parted it and Lawrence dropped three feet and landed badly, his ankle turning on a root. He swore and grabbed the tree trunk.

  "How bad?"

  Lawrence put weight on it and his jaw went tight. "It's fine. Keep going."

  It wasn't fine. Rowan could see the ankle already thickening above the boot. But three more thornbacks came through the undergrowth in a loose pack, drawn by the fire and the shrieking of the burning ones, and Lawrence was already moving.

  Rowan cast Incendio with his right hand and the fire fanned across the ground in front of them. The thornbacks stopped at the edge of the flame line. One tested it with a foreleg and withdrew. The other two circled.

  "They're circling," Iris said. She threw fire at one that got too close on the left and it flinched back. "Rowan, they're going around the flames."

  He could hear more clicking from the upslope. The den was deeper than Thornmane had suggested. "Downhill. We need to keep moving downhill, keep the fire behind us."

  They moved. Lawrence limped and didn't slow down. Iris covered the left flank, throwing fire at anything that got within fifteen feet, her casting getting ragged as her reserves dropped. Rowan covered the right and the rear. He was casting with his right hand because his left was useless, the numb fingers locked around his wand but unable to direct it, and the wand work was clumsy from the wrong hand.

  A thornback lunged from behind a fallen log. Rowan's Flipendo was late and weak and it only staggered the creature. It came on. He hit it with Confringo at ten feet and the blast knocked him backward and showered him with fragments of chitin and something wet that smelled like turned earth and rotting leaves.

  Iris grabbed his arm and pulled him up. "You're bleeding."

  A spine from the thornback had punched through his sleeve below the elbow. He looked at it. An inch of black barb sticking out of his forearm, the skin around it already swelling and hot. He pulled it out. It came with a sound he didn't want to think about and blood ran down to his wrist.

  "Not now," he said. "We need to keep moving."

  More clicking ahead of them now. The thornbacks had circled and cut off the trail they'd used coming up. The fire kept the ones behind them at bay but the ones ahead weren't retreating. They were sitting in the undergrowth communicating, the rhythm of the sound too regular to be random.

  "They're pack hunters," Iris said. She was breathing hard and her wand arm was shaking. "The ground ones drive prey. Poppy's guide said they drive prey into the ambushers in the trees."

  "We're almost past the treeline. There aren't any—" Rowan stopped. He looked at the ground. The web here was thicker than what they'd walked into, the silk strands as wide as his thumb, layered and overlapping. Not a hunting web. Something structural.

  A low vibration ran through the ground. He felt it in his boots.

  The earth shifted. Fifteen feet ahead of them, the leaf litter humped and cracked and something pushed through. Bigger than the others. Much bigger. The body was the size of a small horse, the armour plates along its back ridged into a crest that bristled with spines longer than Rowan's forearm. It pulled itself out of the ground in sections, legs unfolding one after another, eight of them, each one as thick as Rowan's wrist. The eyes were different from the smaller ones. Fewer. Larger. They caught the firelight and reflected it back amber.

  It looked at the three of them and its mandibles spread wide and something hissed between them, a vibration that Rowan felt in his chest.

  "That's a matriarch," Iris whispered. "They're supposed to be rare. Poppy's guide said—" She looked down. The structural web radiated outward from where the matriarch had surfaced, and they were standing in the centre of it. "Oh."

  Lawrence had already seen it. He was looking at the ground around their feet, at the silk-reinforced tunnels running just beneath the surface, and his face had gone very still.

  The matriarch took a step toward them. The ground crunched under its weight.

  Rowan did the math. His reserves were below half. Iris was worse. Lawrence's ankle was bad and his casting speed was compromised. They had thornbacks behind them, thornbacks in the trees to either side, and a creature the size of a horse in front of them.

  He didn't think about it for long.

  "Lawrence, fire behind us, as big as you can. Iris, with me."

  Lawrence planted his feet, favouring the good ankle, and poured everything into an Incendio that roared across the ground web and caught the undergrowth behind them. The silk burned fast and hot and the fire jumped to the pine needles and the dry brush and within seconds there was a wall of flame between them and the pack. Thornbacks screamed. The clicking broke apart into chaos.

  Rowan and Iris hit the matriarch together. His Confringo detonated against its head plates. Her Incendio washed over its back. The matriarch didn't flinch. The armour held. The fire ran off the plates like water and the Confringo left a scorch mark and nothing else.

  The matriarch charged.

  It covered the distance in two strides. Rowan threw himself left and Iris went right and the matriarch went through the space where they'd been standing and tore a furrow in the earth with its claws. It turned faster than something that size should have been able to and came back at Rowan, mandibles wide.

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  He put a Protego between them. The mandibles hit the shield and bit down and the shield shattered. The force of it threw him onto his back. He rolled and a claw slammed into the ground where his chest had been.

  Iris was casting from the side. Incendio, Flipendo, Stupefy, everything she had. The fire didn't penetrate the armour. The Flipendo rocked it sideways. The Stupefy bounced off the head plates. But each spell made it turn its head, tracking the source, and each turn gave Rowan a half-second.

  He got to his feet and looked at the matriarch and thought about the ambusher Iris had killed. Pale belly. The armour was on top. The underside was soft.

  "Keep it turning!" he shouted at Iris, and ran toward the matriarch.

  She hit it with a Flipendo from the left. It swung its head to track her. Rowan dropped flat and slid on the torn-up earth and fired a Confringo into the matriarch's underside from three feet away.

  The explosion was wet and immediate. The matriarch's belly ruptured and it collapsed with its legs buckling outward and its weight came down toward Rowan and he threw up a shield and pushed with everything he had and rolled sideways and the body hit the ground beside him close enough that a spine raked across his back through his robes.

  He lay on his stomach and breathed. The earth was shaking from the matriarch's death throes, legs still working, but the movements were slowing. The clicking in the treeline changed pitch. Something in it that sounded confused.

  "Rowan." Iris's voice. "Get up. The fire's spreading."

  He got up. Lawrence's wall of flame had done its job too well. The dry undergrowth was burning freely and the fire was moving through the treeline on both sides of them. Smoke thickened the air and the mist took it and turned everything grey and orange.

  "Downhill," Rowan said. "Through the fire line before it closes on us."

  They ran. Lawrence's ankle made him lurch with every step and Iris kept pace beside him with a hand under his elbow. Rowan went first, throwing Protego ahead of them when the smoke got too thick, the shield pushing hot air aside enough to breathe. A thornback stumbled out of the burning undergrowth directly in their path, disoriented, its exoskeleton smoking. Rowan hit it with Flipendo and it went sideways and they kept going.

  The fire thinned. The broadleaf forest was greener, wetter, and it didn't burn the way the pine litter had. They broke through the smoke line into cool air and kept running until the ground levelled and the clicking was behind them and the only sound was their own breathing and the distant crackle of the fire they'd set.

  Lawrence sat down against a beech tree and pulled his boot off. His ankle was swollen and purpling. He pressed his fingers around the joint, assessing the damage, and pulled the boot back on without saying anything.

  Iris sat beside him. Her hands were shaking. She held them out in front of her and watched them shake and then put them flat on her knees.

  Rowan leaned against the opposite tree and looked at his forearm. The puncture from the spine was oozing. The skin around it had gone a dark, congested red and the swelling had spread to his wrist. He touched it. Numb. Not the cold-numb of his left hand but the specific deadness of venom.

  "Let me see," Iris said.

  "It's fine."

  She got up and took his arm and turned it in the light. The entry wound was small but the tissue around it was hard and hot. She looked at his face. She didn't tell him it was fine.

  "We need Blainey."

  "We need to not walk into the hospital wing covered in thornback." He pushed off the tree. The world tilted and he put his hand back on the bark until it settled. "Lawrence, can you make it to Blainey if you say you fell on the stairs?"

  "Yes."

  "I've got a bezoar in my trunk. That'll hold until morning."

  "A bezoar is not the same as actual treatment," Iris said. "You know that."

  "I know. But if I walk into the hospital wing with a puncture wound full of venom and a story about falling down stairs, Blainey will have Weasley in her office inside the hour and we won't be going back to the mountains at all." He looked at her. "I'll go to Blainey in the morning if the bezoar isn't enough. I'll make something up. But not tonight, not like this."

  Iris was quiet for a moment. She let go of his arm.

  "If it's not better by morning, I'm taking you myself."

  "All right."

  They started walking. Lawrence's limp was worse after sitting but he didn't ask them to slow down. They'd gone maybe fifty yards when he said, "Next Saturday. We go around the den this time. Bring things that don't cost us reserves."

  Rowan glanced at him. Wrapped ankle, hands raw, blood on his sleeve that wasn't his.

  "We're telling Poppy first," Iris said. "She knows more about thornback behaviour than the three of us put together, and she's going to figure out something happened anyway." She ducked under a low branch. "She'll also never forgive us if she finds out we saw a matriarch and didn't tell her."

  Lawrence didn't argue. He just kept walking.

  The way back through the forest was slow. Rowan's arm throbbed with every heartbeat and the numbness crept past his wrist into his fingers. His left hand was still useless from the cold. He carried his wand in his teeth for a stretch when both hands failed him and Iris kept her wand out and watched the trees.

  When they stopped to rest at a stream crossing, Rowan pulled his shirt up and twisted to get his wand on the scrape across his back. Episkey closed it on the second cast, the skin knitting and the bleeding stopping, though the ache stayed. He sealed the puncture wound's entry point too, the surface of his forearm smoothing over, but the tissue underneath was still hot and swollen with venom that a surface charm couldn't reach.

  "That's not going to fix the venom," Iris said, watching him.

  "I know. The bezoar's in my trunk. This just stops the bleeding through my robes before we get back inside."

  They kept walking. Lawrence had found a branch to use as a walking stick and he was keeping pace but his face was tight every time his weight came down on the bad ankle.

  They came out of the forest at dusk. The castle was lit against the darkening sky, windows glowing. Students were visible on the grounds, heading in for supper. Rowan cast cleaning charms on all three of them, best he could manage one-handed, and Iris fixed what he'd missed. Lawrence straightened up and walked without limping through sheer force of will, his face rigid.

  They came through the third-floor passage. The corridor was empty. Lawrence split off toward the hospital wing and Rowan and Iris climbed to Ravenclaw tower.

  Neither of them talked on the stairs. Iris was chewing the inside of her cheek the way she did when she was working something through. On the landing she stopped.

  "One of them screamed," she said. "When it burned."

  Rowan didn't know what to do with that so he stood there.

  "I keep hearing it." She rubbed the heel of her hand across her eyes. Not crying, just tired. "I'm going to go wash up. If your arm gets worse in the night, wake me. I mean it, Rowan. Don't just sit there with a dead arm and decide you can handle it."

  "I will."

  "You won't, but I'm saying it anyway." She went up the stairs.

  Rowan made it to the dormitory. Hector was at supper. He dug the bezoar out of his trunk and ground it against the wound with his teeth and his one working hand. The Episkey had sealed the surface but the venom was still working underneath, and the powder stung when it hit the reopened skin. The swelling didn't stop immediately but it stopped spreading, and after twenty minutes the numbness in his fingers began to recede.

  He sat on his bed and looked at the ceiling. His arm throbbed. His left hand was a cold dead thing at the end of his wrist. He smelled like smoke and pine sap and something acrid that was probably thornback blood.

  Six thornbacks and a matriarch. A section of treeline on fire. And no moonstone.

  Lawrence came back an hour later. Blainey had wrapped his ankle and told him to stay off it for two days. He sat on his bed and pulled out his mother's last letter and read it. Rowan could tell he'd read it many times because the creases were soft and the ink was smudged in one corner where a thumb had rested.

  "Arm?"

  "Bezoar's working. The swelling stopped."

  "Good." Lawrence folded the letter. He didn't put it away. He turned it over in his hands, running his thumb along the crease. "I was thinking about those structures Thornmane mentioned. Past the ridge. If something was built up there, the site would have centuries of ambient magic in the bedrock. Moonstone forms faster in those conditions."

  "Could be. He said the centaurs won't go near them."

  "He said a lot of things. He also said the thornbacks were dormant during the day." Lawrence put the letter on his nightstand. He lay back but he didn't close his eyes. "My mum dropped a teacup last week. She told me about it in the letter. Made a joke, said she'd have to switch to pewter mugs." He was quiet for a moment. "She doesn't make jokes about things unless she's trying not to be scared of them."

  Rowan looked at the ceiling. There was nothing useful he could say and he knew Lawrence wasn't asking him to say anything. Lawrence wasn't even really talking to him. He was lying in a dormitory bed a hundred miles from his mother talking to the dark because there was nobody else to hear it.

  Hector came in from supper talking about the Ravenclaw-Hufflepuff Quidditch match and who was going to play Seeker and whether Imelda Reyes was really as fast as people said. He stopped when he saw them.

  "What happened to you two?"

  "The library," Lawrence said.

  Hector looked at Rowan's arm, still visibly swollen under the sleeve, and at Lawrence's wrapped ankle. He looked like he wanted to ask a follow-up question. Then he picked up a Quidditch magazine instead.

  Rowan blew out the candle and lay in the dark. Through the tower window the sky was clear and the moon was three-quarters full and the mountains beyond the forest were black against the stars. Somewhere up there a section of treeline was still burning. Somewhere higher, past ridges they hadn't reached, there were structures that centaurs wouldn't approach and moonstone that might be decades from forming or might be sitting in a clearing waiting for someone willing to go far enough to find it.

  His arm pulsed. He let it.

  Next Saturday.

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