“The phone is just a case. Any device would do. Phones are just the easiest and least conspicuous.”

He removed the back of the phone and showed me the contents. Inside, where all the circuitry and computery bits were supposed to be, there was a small battery and two wires joined in the middle by some black electrical tape. He pried this up very, very carefully, and waved me in closer to look. There, wrapped in the fine ends of the wires, was a small stone of some kind—a pinkish one, with a twisting streak down the middle.

“That’s a diamond,” he said.

“You have phones full of diamonds?”

“One diamond each. These wires run a current through it. When we press the one and the nine at the same time, the current runs through the diamond and it emits a pulse that we can’t hear or feel, but it . . .”

“Explodes ghosts.”

“I prefer to think that it disperses the vestigial energy that an individual leaves behind after death.”

“Or that,” I said. “But diamonds?”

“Not as strange as it sounds,” Stephen replied. “Diamonds make excellent semiconductors. They have many practical uses. These particular three diamonds are highly flawed, so they aren’t really valuable to most people. But to us, they’re priceless.”

He carefully snapped the cover back onto the phone. Once he had made sure the phone was closed correctly, he handed it to me.

“They have names,” he went on. “This one is Persephone.”

“The queen of the underworld,” I said. I used to have a book about myths when I was little.

“Described by Homer as the queen of the shades,” Stephen said, nodding. “The one Callum carries is Hypnos, and the one I carry is Thanatos. Hypnos is the personification of sleep, and Thanatos is his brother, death. They get the poetic names for a reason. All secret weapons have code names for the files. What I’ve just given you is an official secret, so please be careful with it.”

I looked at the phone in my hand. I could still smell that smell from the Tube tunnel. I could still feel that wind, see the light . . .

“Does it hurt them?” I asked.

“I have no idea,” he replied. “That question has bothered me in the past, but not now. You need to take that, and if the time comes, you need to use it. Do you understand?”

“I’m never going to understand this,” I replied.

“One and nine,” he said. “That’s all you have to remember.”

I swallowed hard. There was still a burning in my throat from the vomiting.

“Go on,” he said. “Try to get some rest. I’ll be right here. Just keep that with you.”