Works on ghosts

"Sex vampires, I know," I muttered. "Still. It's interesting."
"Interesting?" Michael sounded skeptical. "Harry, I wouldn't call it interesting."
"Why not?" I said. I squinted after Thomas, thoughtfully. "Whatever he used, it worked on the Nightmare. Caught it up. That means that it's either some kind of ambient magic, maybe that cold I felt, that works on everything around, or else it's something chemical - like Red Court venom. Something that got to Lydia's body and bypassed the Nightmare's control of her mind, altogether. Pheromones, maybe."


Grave Peril Chapter 32, Page 355-356