They left before dawn. Five of them this time.

  Poppy had packed a field bag that weighed more than she did. She'd brought specimen jars, ink and parchment for field notes, a set of brass callipers she'd borrowed from Howan, and a small glass instrument Rowan didn't recognise that she said measured ambient magical resonance in biological material. She carried all of it without complaint and kept pace with Lawrence, who was walking on his ankle as though it had never been injured, though Rowan noticed he favoured it on uneven ground when he thought nobody was looking.

  Edmund had brought food. Enough for all five of them, wrapped in cloth napkins from the kitchens, along with a flask of hot tea charmed to stay warm. He'd also brought a pair of thick leather gloves and a coil of rope, neither of which anyone had asked him to bring.

  "Poppy said the mourning colony might still have active webbing on the surface," he said when Iris looked at the gloves. "I'd rather not touch it bare-handed."

  Iris had brought her mapping kit. Improved since the first trip, with a compass she'd charmed to account for the magical interference in the highlands and a set of graduated rulers for distance calculation. She'd spent three evenings preparing it and hadn't told anyone except Rowan, who knew because he'd watched her work on it in the common room while pretending to read.

  They crossed the grounds in the dark. The forest swallowed them. Rowan led, following the route from last week, corrected by Iris's updated map. Lawrence walked beside him and didn't talk. Poppy was asking Edmund quiet questions about what the thornback clicking had sounded like, and Edmund was doing his best to reproduce it by tapping his fingernails together, which Poppy said was completely wrong but useful as a starting point.

  The treeline came up faster than Rowan remembered. The fire they'd set had burned a wide swathe through the pines, maybe two hundred yards across, the trunks blackened and the undergrowth reduced to ash and new growth. The air still smelled faintly of char. The ground web was gone, melted or burned away, and the earth between the roots was bare and dark.

  Poppy stopped at the edge of the burned area and crouched down. She ran her fingers through the soil and brought them to her nose.

  "The silk's gone. Completely. The fire destroyed the entire surface network." She stood and looked deeper into the burn scar. "The underground tunnels might still be intact, though. Thornback silk has different properties below ground. Higher moisture content and more resistant to heat."

  "Will that be a problem?" Edmund asked.

  "Depends on whether the colony has sealed the tunnel entrances or left them open. If they're mourning, they'll have sealed them. If they've already started restructuring around a new queen candidate, the entrances will be open and they'll be aggressive about the territory." She adjusted the strap of her field bag. "I'll know within the first ten minutes."

  Rowan looked at the burned treeline. The matriarch's den was somewhere in there, under the blackened earth. "We'll go around. The route Poppy mapped avoids the thornback territory on the western slope. Poppy, you and Edmund stay here. If anything moves underground, leave."

  "I know what to do if they surface," Poppy said. Her voice had the patient tone of someone who'd been told the same thing three times.

  "I'm telling Edmund."

  Edmund looked at the den and then at Poppy and made a face that said he was already regretting volunteering for this part. "If anything comes out of the ground, I'm carrying her."

  "You are absolutely not carrying me. I'll be documenting."

  "You'll be documenting while I carry you."

  Rowan left them to it.

  The three of them climbed. Poppy's route was better than what they'd had last week. She'd marked plant species on the map that thornbacks avoided, and the path she'd plotted took them through a belt of juniper and rowan that grew along the western face of the ridge. The going was still steep but the vegetation gave them handholds and the ground was drier than the pine litter had been.

  The warming charms held longer this time. Rowan had layered two, one on his gloves and one on his core, and the cold at the treeline was manageable. His left hand was still weaker than his right and the fingers went stiff in the wind, but the potion was finished and the nerve damage was healing. He could grip his wand properly with either hand now.

  They passed the shelf where they'd found the first moonstone readings. Lawrence glanced at it as they climbed past and kept going.

  The caves Iris had marked on the first trip were on the southern face of the second ridge, cut into exposed rock at a point where the heather gave way to scree and bare stone. They looked more deliberate from this angle. The spacing was regular, the openings roughly the same size, the stone around them showing tool marks that the weather hadn't quite erased.

  "These were carved," Iris said. She traced the edge of the nearest opening with her fingers. "Look at the chisel marks. They're consistent. Someone cut these with tools, or with magic that worked like tools."

  Lawrence was already inside the first cave. It was shallow, maybe ten feet deep, the walls rough but the floor flat and swept clean by wind. Nothing inside except rock and a thin layer of dust.

  This book's true home is on another platform. Check it out there for the real experience.

  The second cave was the same. The third was deeper, narrow at the entrance but opening up inside, and at the back of it Lawrence stopped.

  "Rowan."

  Rowan came in. The cave widened into a chamber, low-ceilinged, the rock overhead stained dark with something that might have been soot or something older. Along the back wall, the stone had been carved flat and smooth, and cut into the smooth surface were runes.

  Rowan crossed to the wall and held up his wand for light. The runes were large, each one the size of his spread hand, and they ran in a horizontal line across the width of the chamber. He recognised some of them. Isa for ice and stillness. Hagalaz for disruption. Perthro, the rune of secrets and things hidden. Others he'd never seen before, symbols that resembled no system Fenwick had taught.

  "Some of these are Elder Futhark but used in combinations I haven't seen. The rest are something else entirely." He held his hand near the wall without touching it. A low hum, not audible, something that registered in his chest and the back of his teeth. He'd felt this before. The dungeon door in first year. The hidden wall on the fifth floor. Magic that had been in the stone so long it had stopped being a spell and become a property of the rock. "There's magic in the stone. Old and deep. Don't touch the carvings."

  Lawrence was already searching the rest of the chamber. Iris had her mapping parchment out and was sketching the symbols, each one drawn with dimensions noted.

  They kept climbing. The terrain above the caves was bare rock and scree, the wind constant and cold. After another hour the third ridge levelled into a shallow valley between two peaks, sheltered from the worst of the wind, and in the centre of the valley stood a structure.

  Rowan saw it before he understood what he was looking at. His eyes kept sliding off it, the way they slid off the wall on the fifth floor, as though the stone itself resisted attention. When he forced himself to look directly, the details came into focus: walls of fitted black stone, a single entrance facing east, no windows. Small, roughly the size of a large room. The same material as the ruin he'd found in the forest during first year, but this was intact. Every joint precise. Nothing crumbling.

  The magic here was heavy. He felt it from fifty yards away, a weight in the air that made his ears pop. The vault's magic had been cold and focused. This was something else, saturated, as though the stone had been accumulating ambient energy from the landscape for longer than Rowan wanted to think about.

  Lawrence was already moving. He'd pulled out his wand and was casting the detection charm on the exposed rock before Rowan or Iris had taken more than a few steps toward the structure. Iris ran the same charm from a different angle and looked at her parchment.

  "The resonance here is ten times what we measured on the ridge last week," she said.

  Lawrence didn't respond. He was working the ground around the structure systematically, checking exposed rock faces, splitting likely stones with his wand, examining each one. Rowan and Iris spread out and did the same, covering the valley floor in a grid pattern while the wind blew over the ridgeline above them.

  Lawrence found it forty minutes in.

  "Here." His voice was flat and controlled in the way it got when he was suppressing something large. He was kneeling beside a fissure in the rock at the base of the eastern wall, where a crack in the stone had allowed water to seep and mineral deposits to accumulate over centuries. In the crack, crusted with feldspar and mica, was a vein of something that caught the light differently from the surrounding rock. Translucent, milky-blue, with an internal shimmer that shifted as Rowan moved his wand over it.

  Moonstone. Ritual grade. Two inches of it visible in the crack and likely more beneath.

  Lawrence sat back on his heels. His hands were shaking. He didn't say anything for a while. He just looked at it.

  Then he pulled a chisel from his bag, a thin tool he'd brought for exactly this purpose, and started working the moonstone free from the surrounding rock. His hands steadied as soon as he started the work. The shaking stopped. Iris held a light over the fissure while Lawrence carved, precise and careful, and Rowan watched the structure.

  He didn't go inside. The entrance was dark and the magic radiating from the interior was dense enough that his diagnostic charms couldn't penetrate it, returning only noise. The walls were unmarked on the outside with no runes, no carvings, nothing to indicate purpose or origin. Just black stone, perfectly fitted, and enduring.

  The caves below had runes he could partially read. This structure had nothing. As though whoever built it hadn't needed to write anything down.

  He copied the cave runes into his journal while Lawrence worked. Something for Fenwick. Or for the library, if Fenwick didn't have answers.

  Lawrence worked the moonstone free in three pieces. Each one was wrapped in cloth and placed in his bag with the care of someone handling something irreplaceable. When he finished, he sat on the ground beside the fissure and pressed his palms flat on his knees and looked at the sky.

  "Thank you," he said. He wasn't talking to anyone in particular.

  Iris put her hand on his shoulder. She left it there for a while and didn't say anything.

  They started back down. The return was easier, the route mapped, the thornback territory avoided. They collected Edmund and Poppy at the treeline and found them sitting on a fallen log surrounded by spread parchment and specimen jars. Poppy's notes covered six sheets. Edmund was holding a jar containing something that looked like a strand of silk suspended in clear liquid, and he looked slightly ill.

  "The colony is in deep mourning," Poppy reported as they walked. "Every tunnel entrance I found was sealed with fresh silk. The clicking has stopped completely. I think they're down to about fifteen individuals, maybe fewer. Without a queen candidate, they'll start dispersing within the month."

  "Good," Lawrence said.

  "Good for us. Terrible for the colony." Poppy held up one of her sketch sheets, which showed a cross-section diagram of a thornback tunnel network. "The web architecture is extraordinary. The structural silk they use underground has a tensile strength higher than anything I've read about. I've never seen field data like this."

  Edmund looked at the specimen jar he was carrying and then at Poppy. "Can I put this down now?"

  "Careful with it. That sample took me an hour to extract."

  They walked back through the forest as the light faded. Lawrence was quiet but the silence had a different quality than it had carried all term. Lighter. Something had changed in the set of his shoulders.

  The castle appeared through the trees as the sun set. Warm light in the windows. Smoke from the kitchens. Rowan looked at it and thought about the runes in the caves, the structure in the valley, and the dark archway he hadn't gone through.

  They went in through the third-floor passage, cleaned up, went to supper, and nobody asked where they'd been.