Be realistic: if she won this fight, Old Faithful would be destroyed, and the blood pact would not matter. And if, as was infinitely more likely, she lost … well, then nothing would matter.

She rolled up her sleeve. It was something she would never normally do in front of students, but it was hardly worth worrying about now. Old Faithful raised its eyebrows at the stark colours of Walden’s tattoos. The lowest band of black-and-scarlet spellwriting circled her forearm a little way above her wrist. Walden wore long sleeves all year round to cover the designs, partly because the parents who sent their offspring to boarding school tended to be a conservative bunch, and partly because she had no interest in discussing them with children. She did not discuss them now.

“I give my solemn oath,” she said, “that if you let these two children, Nicola Conway and Mathias Wick, go freely and unmolested from your domain, and never again attempt to harm or distress them in any way, then I will remain here to face you in combat, the outcome to be freely determined between us. And I seal this oath,” she nicked her wrist and held it out, “with my own blood.”

Old Faithful came towards her. Its footsteps were light on the stone flags of the kitchen. It was wearing battered old Converse trainers, and one of its shoelaces was undone. Walden tried not to notice. She stared the demon down, and did not flinch when it grasped her hand, swiped chilly fingers across the cut at her wrist, and then stuck those fingers in its mouth. “Mmm,” it said, pulling them out again with a pop. “Deal. Off you go, kiddos. Door’s unlocked.” It stepped theatrically to one side. The kitchen door behind it swung open, revealing the front hall of School House and the doorframe at the far end. The view through the doorway was dark, but not the roiling dark of the demonic plane. It was nighttime out there, back in the world, the real world, where Walden would almost certainly never set foot again.

She was terrified. But she could not sound terrified. “Go on,” she said.

“Dr Walden—” said Nikki. Mathias gave her a big-eyed look. Neither of them moved.

“Don’t worry, I’ll be right along,” Walden lied ruthlessly, and when that didn’t seem to work, “You’ll only be in my way.”

Old Faithful chuckled.

“Go,” Walden said, putting all the authority she could muster into her voice. “Tell Marshal Kenning—”