“Run us through how the machine works,” Theo said. “Laypeople’s terms.”

He blushed. “It was . . .” He cleared his throat as if embarrassed. “It was explained to me in a song when I was little, to help me remember. They didn’t want to write it down.”

“Could you give us the lyrics?” Petra said.

Hugo cleared his throat and stared up at the shed’s metal ceiling, a flush riding high on his cheeks. “Ley line to Cornerstone; keep the magic calm. Cornerstone to feed the ward; keep the city safe. Machine to light the city; make the demon seen. Plasma flash to bring her low; eliminate the scourge.”

Theo blinked. “Went a little dark at the end there.”

“That last line’s a little harsh,” Hugo agreed. “When I was a kid, I thought ‘scourge’ meant ‘dirt’ or something.”

“Demons are harsh,” I said. “And the song nailed it. The light from the machine turned her kind of green.”

Hugo’s gaze snapped back to mine, curiosity now replacing the embarrassment. “Green?”

“It put a weird cast on her skin. Made it clear she was different.”

“To make the demon seen,” he said again. “Maybe to help the plasma flash—the lightning—identify her.”

“Here’s the problem, Hugo,” Theo began, “which you might have guessed. The demon is powerful. She is a literal agent of chaos. Snaps her fingers, and very bad things happen.”

Hugo looked distraught.

“The machine worked really well,” I reminded him. “The system kept her out for more than a hundred years. Unfortunately, she’s just stronger than the wards. She might have gotten stronger while she waited. And there’s something in Chicago she wants very badly.”

“The Cornerstones,” Petra guessed.

I nodded. “That’s what I’m thinking.”

“How does she know they exist?” Hugo asked.

“That, I don’t know,” I said. “But she’s had a long time to look for them, to puzzle out the possibility.”

“What happens if she gets to the Cornerstones?” Theo asked.

“That would be bad,” Hugo said. “First thing is she could manipulate the wards.” He scratched absently at his arm. “She could turn them off—like unplugging a toaster.”

“Would she have to destroy the stone for that?” Petra asked.

“Or just change the spell.” He pointed to the symbols. “The power of the stone is in the symbols. If she did that, who knows what she could do? Power whatever she wanted using the ley lines, I guess?”

“And that’s why the Cornerstones are magic neutral,” I said, “and why so few people know about them—so they don’t become objects of supernatural lust.

“We need to find her before she gets to one,” I continued, “and we need to stop her. Is there anything else you can tell us about where you think she might go, or where the other Cornerstones might be? Anything that might help us take her in before she hurts more people?”

“No. But if I think of anything, I’ll let you know.”

I didn’t doubt he would. He’d been entrusted with the care of a magical object, and he seemed to take that obligation very seriously. The problem was, based on everything we knew, he wasn’t going to have any more information.