All of this said a lot about the psyche of Harrowhark Nonagesimus, something about Palamedes Sextus, and a little about the mayonnaise uncle, but Gideon was given no time to interrupt. Harrow was continuing, “And I’m not at all sure about the Third. Never mind. Anyway, I’ve spent the majority of my time down the access hatch in the facility. Here.”

Another dry, crackly page was turned. This one was stained with unmentionable fluids and brown patches, which could have been tea and could have been blood. The diagram was much less detailed than the three for the upper levels. In a fat-leaded pencil Harrow had drawn a network of question marks, and some of the rooms were vague sketches rather than the perfectly ruled mazes of the first maps.

Here there were familiar labels: LABORATORY ONE through to LABORATORY TEN. PRESSURE ROOM. PRESERVATION. MORT. WORK ROOM ONE through to WORK ROOM FIVE. And SANITISER, though also: CONTROL ROOM?, CONSOLE? and DUMP ROOM?. It was set out neatly, with corridors all the same width and doors in expected places. It reminded Gideon of some of the oldest parts of the Ninth House, the bits secluded deep below the more modern twisty little hallways and crooked walls with squints.

“It’s very old,” Harrow said, quietly, more to herself than to Gideon. “Considerably older than the rest of Canaan House. It’s pre-Resurrection—or made to look pre-Resurrection, which is just as curious. I know Sextus is obsessed with dating the structure, but as usual, he’s getting caught up in the details. What’s important is the function.”

“So what was it for?”

Harrow said, “If I knew that, I’d be a Lyctor already.”

“Do you know who used it?”

“That’s a much better question, Nav.”

“And why,” said Gideon, “were you down there with your ass kicked to hell, hiding in a bone?”

The Reverend Daughter sighed heavily, then had a fit of coughing, which served her right. “Whoever left the facility also left the majority of their work behind and intact. No theorems or tomes, unless they’ve been removed—and I doubt Teacher removed them—but, as I’ve discovered, it’s possible to trigger … tests. Theorem models that they would have used. Most of the chambers down there were used to prepare for something, and they were left in a state where anyone who comes across it can re-enact the setup. Someone left—challenges—down there for any necromancer talented enough to understand what they were doing.”

“Stop being opaque, Nonagesimus. What do you mean by challenges?”

“I mean,” said Harrowhark, “that I have lost one hundred and sixty-three skeletons to a single laboratory construct.”

“What.”

“I’m prevented from seeing whatever destroys the skeletons I raise,” came the terse answer. “I haven’t worked out how to properly outfit them yet. If the priests have managed to engineer a scaffolded skeleton of the type they use as servants—my God, Nav, have you seen the bonework on them?—then I surely can, but I haven’t worked out how to disassemble one of the First House corpus yet and I can’t do enough just by looking. Don’t get me wrong; I will. I get closer every day. You found me when I’d exhausted myself, that’s all.”

“But what the hell’s it all for?”

“As I have repeated to excess, Griddle, I’m still working on the theory. Nonetheless—look back at the maps.”

The necromancer fell to brooding, staring through swollen eyelids down at the journal. Somewhat astonished still, Gideon leaned over and, ignoring her adept’s dumb mystic despond, flipped the pages back to the three-level plan for Canaan House. A few of the X-marked doors were circled with scratchy black ink and marked with crabbed symbols that she did not recognise. These seemed to be distantly distributed throughout the First House building, tucked away or secreted.

Gideon flipped another page. There was a pencil sketch of an animal’s skull with long horns. The horns curved inward into points that almost touched but not quite, and the sockets were deep holes of black pencil lead. An electric thrill of recognition ran through her.

“I’ve seen this before,” she said.

Harrow bestirred herself. Her eyes narrowed. “Where?”

“Hang on. Let me look at the map again.” Gideon flipped back and found the atrium; she traced with her finger the twisty route from there to the corridor and stairs that led to the cavalier’s dais. She found the staircase, and jabbed with her thumbnail: “You haven’t got it—way ahead of you, Nonagesimus. There’s a hidden hallway here, with a locked door.”

“Are you certain?” Now Harrow was well and truly awake. At the answering nod she rummaged in her robes for a long iron needle and jabbed it inside her mouth—Gideon winced—before the bones at the bedhead unceremoniously shoved her up to a ninety-degree angle, weapon held ready, end shining with red blood. She said, “Show me, Nav.”

Thoroughly satisfied with herself, Gideon placed her finger next to the enormous door of black stone she’d hidden behind the tapestry. Harrow marked the place with a bloody red cross and blew on the ink: it skeletonised immediately into a tarry, dry brown. X-203. The necromancer could not hide a triumphant smile. It stretched her mouth and made her split lips bleed. The sight was incomparably creepy. “If you’re correct,” she said, “and if I’m correct—well.”

Exhausted by all the effort, Harrow closed the journal and tucked it back inside her robe. She sank back down into the dusty embrace of the bones, wrist joints clacking as they lowered her onto the dark slippery material of the duvet. She groped blindly for the water and spilled half of the remnants down her front as she took gulping, greedy sips. She dropped the empty glass onto the bed next to her, and then she closed her eyes. Gideon found herself gripping the slender rapier at her hip and feeling the heft of its basket hilt.

“You could’ve died today,” she said conversationally.

For a long time the girl on the bed was supine and silent. Her chest rose and fell slightly, evenly, as though in sleep. Then Harrow said without opening her eyes, “You could attempt to finish me right now, if you liked. You might even win.”

“Shut up,” said Gideon, flat and grim. “I mean that you’re making me look like a disloyal buffoon. I mean it’s your fault that I can’t take being your bodyguard seriously. I mean that all this sacred duty do exactly as I say blah blah blah shit does not matter in the least if you die of dehydration in a bone.”

“I wasn’t about to—”

“Baseline standard of a cavalier,” said Gideon, “is you not dying in a bone.”

“There was no—”

“No. It’s Gideon Nav Talking Time. I want to get out of here and you want to be a Lyctor,” she said. “We need to get in formation if that’s going to happen. If you don’t want me to ditch the paint, this sword, and the cover story, you’re taking me down there with you.”

“Griddle—”

“Gideon Nav Talking Time. The Sixth must think we’re absolutely full of horseshit. I’m going down there with you because I am sick of doing nothing. If I have to wander around faking a vow of silence and scowling for one more day I will just open all my veins on top of Teacher. Don’t go down there solo. Don’t die in a bone. I am your creature, gloom mistress. I serve you with fidelity as big as a mountain, penumbral lady.”