Kenning spoke up. Walden had forgotten she was still there. In a flat voice she said, “That was at least a ninth-order archdemon. If it had succeeded in forcing an incursion, it would have eaten you all alive, followed by the rest of the school. The main reason it didn’t succeed is that you are lucky enough to be taught by one of the most skilled invokers in the country. If someone under my command made a mistake that stupid, I would have him kicked out of the Order.”

Will’s mouth worked. He said, “Oh.”

That was a little harder than Walden had meant to hammer him, but it seemed to have worked. “Think about that,” she advised. “I think you owe Mathias an apology. Do I need to call you both into my office, or can I trust you to speak to him yourself?”

Will said, “I’ll do it.”

Walden made a mental note to follow that up later. Will was almost never actively malicious, but he could be slippery, and he tended to conveniently forget anything that he would find difficult or embarrassing to deal with. “And I will look forward to reading your practical report next week,” she said. “It goes without saying that I expect a clear analysis of exactly what went wrong today. Now, off you go.”

When Will was gone, looking chastened, she said, “Well. Thank you for your help this morning, Laura. Shall we go and get a cup of tea?”

“I need to write up this incident,” said Kenning. She still looked annoyed. “So do you, don’t you? That boy should be punished. Detention, at least.”

If Walden thought that a lunchtime detention would have the slightest effect on Will, she would have given him one. But he was the sort of boy who laughed off official punishments, or even boasted about them, and then forgot they had ever happened. Shame, on the other hand, would stick with him. “I assure you,” she said, “having to apologise to someone, and then write a report explaining that he got something wrong, will be much harder on him. If you’ll leave the discipline to me, please.”

“What was an archdemon that size doing here?” Kenning said. “That thing was at least ninth order. It was intelligent! Why didn’t I know about it?”

“Tenth order,” said Walden, stifling a sigh. “Or possibly eleventh. And I’m sure you did know about it. The previous Chief Marshal must have mentioned Old Faithful.”

“Old Faithf—yes, as a joke to frighten the newbie!”

“Not a joke, I assure you,” Walden said. “Old Faithful has been hanging around Chetwood School since the seventeenth century at least. It’s fairly quiet but it makes an occasional push.”

“It’s been here for three hundred years? Why has no one killed it?”