But of course there was no need to tell Laura anything. She was good at her job. Walden’s plan for this fight was to spin it out as long as she could and pray her two hours ran out before Old Faithful managed to possess her. Incursion collapse would seal them both in here, and then it wouldn’t matter in the least who won. Walden had a pretty clear idea of the odds. Hopefully Nikki and Mathias coming out of the incursion with the story of what Walden had agreed to would prompt Laura to act sooner.

With a certain amount of chivvying, the sixth formers finally set off down the corridor. Walden watched them go with her heart in her mouth. If she’d made a mistake—if Old Faithful went back on the deal now—

“Don’t worry,” said the demon, who was standing unexpectedly at her elbow. Walden could not suppress a sudden, awful startle. “I don’t need them anymore. It’s you I want.”

Walden backed away from it. Her wrist still hurt where she’d nicked it. She started unbuttoning her blouse. “Oh, wow,” said Old Faithful. “This is sudden, Saffy. Can’t we make out first?” Again Charlie’s engaging grin. “I didn’t want to die a virgin. It just happened.”

Walden finished with her blouse and shrugged it aside. Underneath, because it was October, she was wearing a sensible vest. The demon’s eyes moved over the tattoos on her arms; bands of spellwork on both, creeping florals woven around them on the left, the exuberant reds and oranges splashed across Walden’s right bicep. “Those are new,” it said.

“Not really,” said Walden.

“Sure we can’t kiss? For old times’ sake?”

“I am thirty-eight years old,” Walden said flatly. “I have no interest in being pawed by a monster wearing the corpse of a child.”

“You really are no fun at all,” said Old Faithful.

“I’m a teacher, Charlie,” Walden said. “I’m the opposite of fun.”

The demon smiled at her. Walden heard her own slip a moment late: Charlie. This wasn’t Charlie, had never been Charlie.

“When I’m you,” Old Faithful said, “I’m going to walk out there and they won’t know.”

Walden said nothing.

“I’m not some pathetic little imp who hides in a photocopier and eats biscuits. I can think. I can lie,” the demon said. “And I’ll know everything you know, because I’ll have your brain. It’ll be my brain. Your doctorate will be my doctorate. Your office will be my office. And I’ll call the naughty children into my office, and I’ll have a little snack.” Charlie’s crooked grin got broader. “Just a little snack. A little here, a little there. I won’t get caught. I’ll get stronger and stronger. And there’s nothing you can do.”