It was not the incursion ward preventing Walden from entering. A badly drawn or overgeneralised ward might accidentally exclude humans as well as demons, but this one was written with Nikki’s characteristic precision and verve. The identity clause directing it against any unwelcome demon was perfect to the last dot and curve. There was nothing to find fault with at all, in fact, except that there was not enough power in it to present a serious barrier to anything larger than an eighth-order archdemon. Even that was a lot of power for a sixth former. Mathias was in there too. He had to be. Nikki could not have put that much force into a ward alone.

So something else was barring Walden’s way.

She frowned at the door. It took her a few seconds to spot the subtle, smoky shapes of demonic spellwork. Demons very seldom actually cast spells, in the same way that birds seldom needed aeroplanes. The logic and formality of spellcasting were an intellectual step too far for most of them anyway. But Old Faithful was no ordinary demon. The spell barring Walden’s way forward was a subtle and powerful piece of magic. Again she felt the researcher that she no longer was clamouring for her attention: If only you had time to take notes!

She glared at the demonic barrier. It was a spell and therefore it would be possible to analyse it and unravel it. The smoky shapes tried to shift away from her gaze. “No, you don’t,” Walden said, pinning it in place with a glare. The spell had a location clause, she saw, but no weaknesses there—though it defined School House in confusingly roundabout terms, including a timeframe element that ran backwards as well as forwards, which had startling implications. And the identity clause—

Ah.

Those who do not belong, said the demon’s spell. Those who have no right to enter. Unforgivably vague, if a human had written it, but Old Faithful had more than enough power to hold its concept of not-belonging firm without sacrificing overall spell stability.

Nevertheless, it was an opening.

“I am the Director of Magic at Chetwood School,” said Walden. “I am a senior member of school staff, undertaking safeguarding duties in accordance with my professional obligations as outlined in my employment contract. There is nowhere on the school grounds where I have no right to go, particularly if I believe a child is in danger. And your incursion seems very dangerous, my friend.”