At this point Walden intervened to check their work, because there were limits to self-directed learning, and ‘actually summoning a demon’ was one of them. While she was making minor corrections to the sixth formers’ summoning array, Laura Kenning came in and took up a position in the corner of the lab, arms folded, silent. “Good morning, Marshal,” said Walden in a friendly way, because it was important to model professionalism and good manners in front of the children.

Kenning said, “Morning.” The sword on her hip caught the light from the window, a bright glint of silver. Her cropped blonde hair was a golden halo. She looked like she’d escaped from the school chapel’s stained glass, a butch take on an avenging angel.

The sixth formers gave her uneasy looks.

“All right!” said Walden, snatching back their attention. “Good work, everyone.”

“I have a question,” said Nikki. “What would we summon a fourth-order demon for? I mean, in the real world.”

Ah, the utilitarian question. All teachers got this sometimes, and the real answer—Education is more than a skills checklist for a job application—seldom satisfied most teenagers. Walden could have said: Not everything worthwhile in life is useful. She truly believed that. She could also have said: We are using this esoteric academic skill in the real world right now, and its purpose is to turn you into a person who is undaunted by complex, high-stakes, multistage brainwork. But that wouldn’t have been fair to the spirit of Nikki’s question, so she went for a less abstract answer.

“Nothing, I hope,” she said. “There is absolutely no piece of magic you can do by summoning a demon this size which you could not also do in some other, much safer way.” This was untrue, she realised as she spoke. Having Kenning in the room was putting her on edge. Never enjoyable to be observed teaching, and particularly unpleasant to be observed by a hostile person carrying a sharp object. “I beg your pardon, I misspoke. What I meant to say is: there is no piece of magic you should do which can only be done with the assistance of a demon. If you remember our magical ethics topic from last year.”

“There’s military stuff,” said Will. “My uncle does it.”

And absolutely should not have mentioned it to you, Walden thought, but she said, “Yes. There is, as you say, military stuff.”

Aneeta said, “Did you ever, like—do it?”

“I was approached by the Pentagon when I lived in the United States,” said Walden. “I told them no.”