“Yes,” said Harrow. “Congratulations to you also, Warden.”

He pushed the door open onto total blackness. Harrow stepped closer to Gideon and muttered, “If anything moves—”

“Yaaas, I know. Let it head for Camilla.”

Gideon did not know how to handle this new, overprotective Harrowhark, this girl with the hunted expression. She kept looking at Gideon with the screwed-up eyes of someone who had been handed an egg for safekeeping and was surrounded by egg-hunting snakes. But now she stepped forward grandly, spread her palms wide in the necromantic gesture as threatening as a cavalier unsheathing a sword, and strode into the dark. Palamedes went after her, groped around on the wall for a few moments, and then hit the light switch.

Gideon stood in the laboratory and stared as Camilla carefully closed the door behind them. This Lyctoral lab was an open-plan bomb wreck. There were three long lab tables covered in old, disused tools, splotches of what looked to be russet fungus, abandoned beakers, and used-up pens. The floor underfoot was hairy carpet, and in one corner there was a hideous, slithery tangle of what Gideon realised must be sleeping bags. In another corner, an ancient chin-up bar sagged in the middle alongside a strip of towel left to hang for a myriad. Everywhere there were bits of paper or shaken-out clothes, as though somebody had left the place in a hurry or had simply been an unbelievable slob. Spotlights shone down hot on the ruined jumble.

“Hm,” said Camilla neutrally, and Gideon knew immediately that she organised Palamedes’s and her socks by colour and genre.

Harrowhark and Palamedes picked their way through the mess to the tables. Palamedes was saying in his explanation voice: “It’s not as though I didn’t complete this challenge by lunchtime, though I had a distinct advantage. It was a psychometrical challenge. The main difficulty was working out what the challenge wanted in the first place: it was set up by someone with an obscure sense of humour. It was just a room with a table, a locked box, and a single molar.”

“Reconstruction?”

“Not all of us can respring a body by dint of a molar, Reverend Daughter. Anyway, I must have examined that tooth for two hours. I know every single thing there is to know about that tooth. Mandibular second, deciduous eruption, vitamin deficiency, male, died in his sixties, flossed obediently, never left the planet. Died in this selfsame tower.”

Both of them were riffling through the papers left on the desk: Palamedes left them in forensically exact piles divided by where they had been found. He adjusted his glasses and said, “Then Camilla took over because I wasn’t bloody thinking.”

Camilla grunted. She had meandered over to look at the rust-pitted crossbars of the chin-up, and Gideon had repaired to the worm mound of sleeping bags to kick them unhelpfully. Harrow said impatiently, “Get to the denouement, Sextus.”

“I had tracked the tooth. It told me nothing—no spiritual links to any part of the building. It was a black hole. It was as though the body it came from had never been alive. No ghost remnants, nothing—this is impossible, you understand, it meant the spirit had somehow been removed entirely. So I did some old-fashioned detective work.”

He peered under an abandoned clearfile. “I looked upstairs for the skeleton with the missing upper molar. He wouldn’t come down with me, but he did let me make a plaster impression of his clavicle. The clavicle! Someone was having a joke. Anyway, you can imagine my reaction when I unlocked the box with it and found it empty.”

Gideon looked up from a pasteboard box she had found: it was full of the ring tabs you got on pressurised drink cans, and jingled unmusically when she shook it. “The constructs? Like, the bone servants?”

“Second’s right, first isn’t,” said Camilla laconically.

“They’re the opposite of what Lady Septimus calls the beguiling corpse,” said Palamedes. “They seem to have most of their faculties intact. Mine was very nice, though he’s forgotten how to write. The skeletons aren’t reanimations, Ninth, they’re revenants: ghosts inhabiting a physical shell. They simply lack a true revenant’s ability to move itself along a thanergetic link. The beguiling corpse is a remnant of spirit attached to a perfect and incorruptible body—that’s the idea, anyway—where what I’ll term the hideous corpse is a fully intact spirit attached permanently to a rotting body. Not that someone hasn’t preserved those bones beautifully.”

Harrowhark slammed a ring-binder down on the bench.

“I’m a fool,” she said bitterly. “I knew they moved too well to be constructs—no matter how I tried to mimic how they’d been done. I just could have sworn—but that’s impossible. They’d need someone to control them.”

“They do—themselves,” said Palamedes. “They are autonomously powering themselves. It debunks every piece of thanergy theory I ever learned. The old fogeys back home would peel their feet for half an hour alone with one. It still doesn’t explain why there’s no energy signature on the bones, though. Anyway, this is the laboratory of the Lyctor who created them—and here’s their theory.”

Much like the one back in the other laboratory, the theorem was carved into a big stone slab pinned down in a dusty back corner and covered up with loose-leaf flimsy. Both cavaliers drifted over, and they all together stared at the carved diagrams. The laboratory was very quiet and the spotlights haloed streams of dust so thick you could lick them.

Resting on the edge of the stone set into the table, there was a tooth. Palamedes picked it up. It was a premolar, with long and horrible roots: it was brown with age. He handed it to Harrow, who gently unfolded it in the way that only a bone magician could and in the way that always made Gideon’s jaw hurt. She turned it into a long ribbon of enamel, an orange with the skin taken off and flattened, a three-dimensional object turned two-dimensional.

Written on the tooth in tiny, tiny letters was this:

FIVE HUNDRED INTO FIFTY

IT IS FINISHED!

Harrowhark took out her fat black journal and was scribbling down notes, but Palamedes had abruptly lost interest in the theory stone. He was looking at the walls instead, flipping open some of the ring-binders that she had discarded. He stopped in front of a faded pinboard, riddled thick with pins, all with bits of string attached. Gideon came to stand next to him.

“Look at this,” he said.

There were rainbow splotches of pins all over the board. There were tiny clusters, and Gideon noticed that at the centre of each cluster there was one white pin; the smallest and most numerous clusters had three pins fixed around one white pin. Some others had five or six. Then there were two other separate whorls of pins, each made up of dozens alone, and then one enormous pin-splotch: more than a hundred of them in a rainbow of colours, thickly clustered around one in white.

“The problem of necromancy,” said Palamedes, “is that the acts themselves, if understood, aren’t difficult to do. But maintaining anything … we’re glass cannons. Our military survives because we have hundreds of thousands of heavily armed men and women with big swords.”

“There’s always more thanergy to feed from, Sextus,” said Harrow distantly, flicking her eyes back and forth as she copied. “Give me a single death and I can go for ten minutes.”