The night watch guard stepped into the cold stone corridor, counting her strays. Six tonight. She wanted a full patrol, every hall covered, every corner watched. But the deeper halls had been spitting her cats back out for a week now. Tonight, she would find the cause. Past the second bell, the dark grew thick. A trail of glowing blue notes drifted down the corridor, humming a tune that made her ears feel stuffed with wool. The strays froze. One of her sleepy-eyed guard cats raised his sword at nothing, swinging at the empty air as if fighting a dream. The notes poured from the decrepit tower at the west end. She pushed inside and saw it. A tall black harp, wrapped in cobwebs, strings trembling on their own. No hand touched them. The music swelled, and her strays bolted as one, tails low, claws skittering on stone. She chased them out into the courtyard. They streamed across the wet grass toward a gnarled tree she had never trusted. Its trunk gaped open like a mouth. One by one, the strays vanished down the hollow into the dark below. The guard knelt at the opening. Cold air breathed up at her, carrying the same faint tune. The roots curled down into a tunnel she could not see the end of. Her drowsy swordsman caught up, blinking hard, finally awake. She drew a slow breath and climbed down after her cats. The harp's song followed her into the earth. Whatever fed those strings, it lived below the castle. And tonight, for the first time, she was going to meet it.

The blue notes drifted past her in the tunnel, humming that sick, twisting tune. The guard clamped her hands over her ears and ran. Her boots pounded the dirt as she chased the last flick of a tail into the dark. The music would not stop. Neither would she. The tunnel spat her out under a huge gnarled tree, its roots curling like ribs around a black opening. At the foot of the trunk lay a jagged pile of tiny swords. Her guard cats had dropped them in pure panic. She picked one up, felt its weight, and set her jaw. She climbed into the hollow. The drop came fast. She slid past twisted roots, scraping her shoulders, until her boots hit cold stone. She stood up in a cavern lit by towering crystals. They pulsed pink and gold in time with the harp song above. Some were split clean through, their cracks weeping faint light. The music was breaking them. She pressed deeper, one hand on the wall. At last she saw them. Her six guard cats, shrunk back into kittens, huddled and shaking. They circled a pillar of dark stone. None of them looked at her. None of them came. The pillar bore Latin scratched deep into its face. A keyhole sat in its crown like a small black eye. She read the words aloud, slow and careful. Her heart has gone cold. Find the keys to open her stone cold heart. The harp's tune slid through the cavern and the kittens flinched as one. The guard knelt and scooped the smallest kitten into her arms. It did not purr. It did not know her. She understood then that no calling, no training, no patient sitting would bring them back tonight. She would have to find the keys. She tucked the little sword into her belt and stood, facing the dark beyond the pillar.

The guard stood with the small sword at her belt and the ring of dark keys cold against her palm. She had pulled them from a hook deep in the tunnel and carried them down without knowing if any would fit. The kittens still circled the dark pillar in a tight, frozen ring, eyes fixed on the keystone. The blue harp song poured down from the hollow above and would not stop. She pressed her hands to her ears and forced herself to look at the cavern instead of the cats. She saw it then. Tall cracked crystals lined the walls, pulsing in time with the song. They were catching the harp's music from above and throwing it inward. At the top of the pillar, a single dark crystal hung in the air without a chain. The wall crystals fed it. The floating one trembled and held the kittens still. That was the lock on them, not the pillar itself. A shadow dropped through the hollow. Whisper Thornpaw landed light on the stones, hood thrown back, tambourine in one paw. The jeweled rim caught the cavern light and rang once as she found her footing. The note slipped into the resonance. The wall crystals flared. The floating crystal pulsed harder, and the kittens broke their circle. They hissed. Their small backs arched. One swiped at the guard's boot. "Put it down," the guard said. "You are making it worse." Whisper lowered the tambourine and crossed to the pillar. She studied the keyhole in the crown. "I have keys like that," she said. She pulled a ring from her satchel. Three keys, plain iron, nothing like the eerie set on the guard's belt. She picked one and slid it into the keyhole. It turned with a soft click. The floating crystal above the pillar cracked straight down its center. The humming cut off. The wall crystals dimmed to a low, ordinary glow. The kittens dropped flat to the stone, shook themselves, and looked up. Six small faces found the guard at once. She knelt. The smallest kitten walked to her boot and pressed its head against her shin. The others came after. She gathered them in against her coat and felt their weight settle. They were still kittens, still small, but they knew her again. She let out a breath she had been holding since the first bell. Whisper stood at the pillar, turning the broken half of the floating crystal in her paw. Behind the keyhole, a narrow seam had opened in the stone. A thin shelf slid out on its own. On the shelf sat a folded sheet of paper, edged in the same blue light the harp had used, and a second keyhole carved deeper in, waiting. The guard looked at the kittens in her arms, then at the new lock. The song was done. The work was not.

The kittens came back up the tunnel one at a time, slow on the stairs, blinking at the courtyard light. By the time the last one climbed out of the hollow tree, they were full-grown again. Their coats settled. Their swords fit their paws. The guard counted six and let her shoulders drop. Whisper Thornpaw stood at the edge of the courtyard with the broken crystal in her satchel, waiting. "Your fine," the guard said. "I am lifting it. Play at the gates whenever you want." She pulled the slip from her coat pocket and tore it in half. Whisper watched the pieces fall. "Cold ground, warm ground," Whisper said. She offered a paw. The guard took it. They shook once. Whisper turned and walked out through the iron gates without looking back. The guard led her six to the stone post by the gates. She lit the tall floor lantern inside the door. Its star-cut panels threw light across the small room. The cats sat in a half circle on the flagstones. "New rule," she said. "There is still music out there. Not the harp. Something else." She made them listen. Past the second bell, a thin line of notes drifted in from the woods, pale and curling, neither close nor far. The cats' ears swiveled toward it together. "You do not chase it alone," she said. "You walk in pairs. You keep each other's lantern in sight. If you find the source, you raise your lantern straight up. The rest of us come to that light." She walked them through the route. She took them along the wall, past the hollow tree, to the edge of the moor where the drifting notes hung thickest in the air. The notes were visible there if you stood still long enough — faint marks moving on nothing, like ink in water. She marked the turning point at the old stones and the turn back at the gate. Two pairs would walk the outer line. One pair would hold the post. She had each cat lift its small lantern overhead until she could see the signal clear from the far end of the wall. The smallest one struggled. She knelt and adjusted his grip until the light stayed steady. At the third bell she called them in. They came at a run, six lanterns bobbing through the dark, and formed up at the post door. She raised the tall lantern from its hook and held it above her head. "Watch is set," she said. The cats answered with a low, even sound, not a meow, something closer to assent. She sent the first pair out. They went without flinching. She stood in the doorway and watched their lights move along the wall, and listened to the music she still could not name.