“It’s not a hold ward,” Dulcinea explained, “but it’s not just physical. The ward will alert me if the immaterial try to pass … if they’ve instantiated, I mean, if they’ve crossed over. I don’t want to stop them,” she added, when Harrowhark started fidgeting with a bone fragment from her pocket. “I want to see whatever would try to sneak in on us … I want to know what it looks like. Let’s go.”

Rather than the neatly sectional space that had constituted Laboratory Two, with its Imaging and Response chambers and orderly empty shelves, Laboratory Eight opened up on an enormous grate. A lattice of thick black steel barred the first part of the room from the second, which—espied through the holes—proved to be a long space with a claustrophobic ceiling. It was like stepping into a pipe. The door led to a metal platform on struts and a short flight of stairs leading down into the space, barred by the huge grate. The Seventh necromancer went to the wall and flicked a switch, and with a low vibrating moan, the grate slowly began to tuck itself up into the ceiling.

With the removal of the grate, the room seemed enormously grey and empty. Only two things broke up the vast monotony of grey metal and white light: far off at the other end of the chamber was a metal plinth, boxed on top with what looked like clear glass or plex; and at the bottom of the stairs, about a metre away from its base, was a yellow-and-black-striped line that had been painted horizontally from wall to wall.

It was easily a hundred metres from the stripe to the plinth: a long way to walk. It looked simple enough, which was how Gideon knew it was probably a huge pain in the ass.

And yet her adept was already gliding down the stairs, standing before the yellow-and-black-emblazoned line as though at the edge of a fire. Dulcinea came after, leaning more heavily on her crutches as she swung herself down the stairs. Protesilaus came last.

“If you put your hand through,” she said, “you’ll see—there.” Harrow had bitten off a cry of pain. She had stuck her gloved fingers tentatively over the line, and now she was yanking off her glove to see the damage. Gideon had been the victim of this once before, through Palamedes Sextus, but it was still a disquieting sight. Harrow’s fingertips had shrivelled: the nails had split horribly, and the moisture looked as though it had been siphoned forcibly out, wrinkling the skin like paper. Her adept shook her hand in the air like you would with a burn; the wrinkles smoothed out, slowly, and the nails knit themselves back together.

“Hardly insurmountable,” said Harrow, having regained her composure.

“Very hopeful! What would you use?”

“A corporeal ward; skin-bound, tight focus.”

“Try it.”

Harrowhark flexed her fingers slowly. Gideon watched as she narrowed her eyes into obsidian slits, fringed thickly with blunt black lashes, and then extended her hand beyond the line again. There was a brief shower of blue sparks; Harrow snatched her hand back, amazed and furious. The fingers had withered into puckered twigs; her little nail had fallen off entirely. The edges of her sleeve had holed and frayed as though assaulted by moths. Gideon lunged out of a sheer desire to do something, but Harrow held her back with her healthy hand, staring fixedly at the hurt one as it slowly mended. Dulcinea watched with eager eyes: Protesilaus hulked next to the stairs.

Harrow shook a bracelet over her hurt hand, and bands of spongy osseous matter wrapped around her knuckles before forming thick plaques of bone. Gauntleted, she reached her hand out again—

“It won’t work,” said Dulcinea, dimpling.

—The gauntlet exploded into fragments of bone. Those that passed the yellow line fragmented further, and those bits degraded into dust and that into powder. The glove fell away in hunks, dwindling into fine sand before it even hit the ground, and Harrow yanked her hand back to stare at its sad puckered appearance a third time. She sat heavily on the stairs, and a bead of blood sweat trickled down one temple as, away from the barrier, her hand relaxed back into wholeness. Gideon longed to say: What the fuck?

“It’s two spells, overlaying each other,” said Dulcinea.

“You can’t have two spells with coterminous bounds. It’s impossible.”

“But true. They’re really coterminous—not just interwoven or spliced. It’s truly delicious work. The people who set it in place were geniuses.”

“Then one half is senescence—”

“And the other half is an entropy field,” said Dulcinea simply.

Gideon followed Harrow’s gaze over the long, dully gleaming field of corrugated metal, and the plinth shining at the end like a beacon. She saw Harrow suck in and bite the inside of one cheek, always a sign of furious thinking, flexing her fingers all the while as though still worried about their integrity. She took an old, ivory-coloured knuckle from her pocket, and she passed it to Gideon. “Throw,” she commanded.

Gideon obligingly threw. It was a good toss—the knuckle hit the field high and travelled for about half a metre before fragmenting into a rain of grey particles. Harrow’s gaze fixed on the crumbling shards: more tiny spikes and spurs of bone burst out of them and shrivelled, stillborn—another burst as Harrow curled her fist into a ball—then nothing. There was no more bone left.

Dulcinea breathed in admiration: “It’s awful quick.”

“Then,” said the adept of the House of the Ninth, “it is—and I don’t say this lightly—impossible. This is the most efficient death trap I’ve ever seen. The senescence decays anything before it can cross, and the entropy field—God knows how it’s holding—disperses any magical attempt to control the rate of decay. But why hasn’t the whole room collapsed? The walls should be so much dust.”

“The field and the flooring are a few micrometres apart—maybe the Ninth could make a very very weeny construct to go through that gap,” said the Seventh helpfully.

Harrow said, in bottom-of-the-ocean tones: “The Ninth House has not practised its art on—weeny—constructs.”

“Before you ask, it’s not a lateral puzzle either,” said Dulcinea. “You can’t go through the floor because it’s solid steel, and you can’t go through the ceiling because that’s also solid steel, and there’s no other access. And Palamedes Sextus estimated you could walk for probably three seconds before you died.”

Harrow got very focused very suddenly. “Sextus has seen this?”

“I asked him first,” said Dulcinea, “and when I told him the method, he said he’d never do it. I thought that was fascinating. I’d love to get to know him better.”

That got every particle of Harrowhark Nonagesimus’s attention. Dulcinea absently tossed her crutches to Protesilaus one by one, and he caught them out of the air as though he didn’t even have to think about it, which Gideon had to admit was cool. She sat down heavily on the stairs quite close to Harrow, and she said: “There is one way of doing it … and he wouldn’t. I’m sorry that I didn’t admit it … but you were my second choice. If black vestals won’t cross this line, I don’t think anyone will. And I can’t, because I physically can’t walk the whole way unassisted. If I faint or go funny halfway there it will mean my timely death.”