She put her hand out and caught her fingers in the spell as she spoke. It twisted away from her, trying to escape—oh, this was not a passive defence; Old Faithful had certainly noticed her, and it was paying attention now. But the principles of invocation relied above all on the inviolability of a bargain; to a demon, the wording of Walden’s employment contract carried as much magical force as any summoning array. Walden closed her fingers on the greasy, smoky heart of the barrier spell. Dr S. Walden, D.Thau, M.Ed, Director of Magic, alumna. Chetwood School knew who she was. She had every right to be here.

The barrier fell apart. The door of Nikki’s bedroom swung open.

Walden could see nothing through it. The fog of demonic magic was thick as the soup from the school canteen. But she heard a low cry of alarm in the distance. Opening the door had disrupted Nikki’s incursion ward, and its white light was fading. Walden stepped over the threshold, slammed the door shut behind her, and sent a firm blast of power into the ward to shore it up. It was much too late to actually prevent this incursion, but every ward still functioning would help to slow its growth. As the spell’s glow grew brighter, the fog around Walden receded a little, revealing that Nikki’s bedroom floor was an absolute tip—shoes, clothes, a ragged-looking stuffed rabbit perched on top of a beanbag, a slipping pile of lever arch files all bursting at the seams, two stray hockey sticks. “Nikki!” Walden called sharply, before she tried to pick her way across. “Mathias!”

There was an answer. Walden recognised Nikki’s voice. It sounded frightened, and very far away.

“Stay where you are!” Walden called back. “I’m coming for you.”

She threw some more power into her upturned pale bowl of a shield, and then for good measure added another layer to it, remembering how neatly Kenning’s shield had reinforced itself. Then she set out across the floor.

Old Faithful, or else the peculiar geography of the demonic plane, was playing tricks. There was much more floor than was logically possible at the top of a tower, and all of it was covered with mess. Nikki couldn’t possibly own this much stuff, and Ebele would never have let her leave all her possessions lying around like this. Out of the corner of her eye, Walden caught sight of a battered blue guitar, decorated with stickers and propped on its amp. The skin on the back of her neck crawled. Neither Nikki nor Mathias was musical.

She began to be afraid that the absurd obstacle course would go on forever. She did not spot the summoning pentagram until she was almost on top of it.