I could practically hear the pride bursting out of his voice: "Got it!" he piped. "Will do, my lord!"

He rolled down a window and buzzed out. I rolled it back up and took a look around the hearse with both normal and supernatural senses, to be sure we were alone. Then I turned back to the skull.

"Bob, it's just you and me talking here. Think about this. Mab sends me off to kill Maeve, something that would be impossible for me to do on my own-and she knew that you know how to do it. She knew the first thing I would do is come back here as the first step in the job. I think she meant for me to come to you. I think she meant for you to tell me."

The skull considered that for a moment. "It's indirect and manipulative, so you're probably onto something. Let me think." A long minute went by. Then he spoke very quietly. "If I tell you," he said, "you've got to do something for me."

"Like what?"

"A new vessel," he said. "You've got to make me a new house. Somewhere I can get to it. Then if they come after this one, I've got somewhere else to go."

"Tall order for me," I said soberly. "You've basically got your own little pocket dimension in there. I've never tried anything that complicated before. Not even Little Chicago."

"Promise me," Bob said. "Promise me on your power."

Swearing by one's power is how a wizard makes a verbal contract. If you break your word, your ability with magic starts to fray, and if you keep doing it, sooner or later it'll just wither up and die. A broken promise, sworn by my power, could set me back years and years in terms of my ability to use magic. I held up my hand. "I swear, on my power, to construct a new vessel for you if you tell me, Bob, assuming I survive the next few days. Just . . . don't expect a deluxe place like you have now."

The flickering eyelights flared up to their normal size again. "Don't worry, boss," Bob said with compassion. "I won't."

"Wiseass."

"Right, then!" Bob said. "The only way to kill an immortal is at certain specific places."

"And you know one? Where?"

"Hah, already you're making a human assumption. There are more than three dimensions, Harry. Not all places are in space. Some of them are places in time. They're called conjunctions."

"I know about conjunctions, Bob," I said, annoyed. "When the stars and planets align. You can use them to support heavy-duty magic sometimes."

"That's one way to measure a conjunction," said the skull. "But stars and planets are ultimately just measuring stakes used to describe a position in time. And that's one way to use a conjunction, but they do other things, too."

I nodded thoughtfully. "And there's a conjunction when immortals are vulnerable?"

"Give the man a cookie; he's got the idea. Every year."

"When is it?"

"On Halloween night, of course."

I slammed on the brakes and pulled the car to the side of the road. "Say that again?"

"Halloween," Bob said, his voice turning sober. "It's when the world of the dead is closest to the mortal world. Everyone-everything-standing in this world is mortal on Halloween."

I let out a low, slow whistle.

"I doubt there are more than a couple of people alive who know that, Harry," Bob said. "And the immortals will keep it that way."

"Why are they so worried?" I asked. "I mean, why not just not show up on Halloween night?"

"Because it's when they . . ." He made a frustrated noise. "It's hard to explain, because you don't have the right conceptual models. You can barely count to four dimensions."

"I think the math guys can go into the teens. Skip the insults and try."

"Halloween is when they feed," Bob said. "Or . . . or refuel. Or run free. It's all sort of the same thing, and I'm only conveying a small part of it. Halloween night is when the locked stasis of immortality becomes malleable. They take in energy-and it's when they can add new power to their mantle. Mostly they steal tiny bits of it from other immortals."

"Those Kemmlerite freaks and their Darkhallow," I breathed. "That was Halloween night."

"Exactly!" Bob said. "That ritual was supposed to turn one of them into an immortal. And the same rule applies-that's the only night of the year it actually can happen. I doubt all of them knew that it had to be that night. But I betcha Cowl did. Guy is seriously scary."

"Seriously in need of a body cast and a therapist, more like." I raked at my too-long, too-messy hair with my fingers, thinking. "So on Halloween, they're here? All of them?"

"Any who are . . . The only word I can come close with is 'awake.' Immortals aren't always moving through the time stream at the same rate as the universe. From where you stand, it looks like they're dormant. They aren't. You just can't perceive the true state of their existence properly."

"They're here," I said slowly. "Feeding and swindling one another for little bits of power."

"Right."

"They're trick-or-treating?"

"Duh," Bob said. "Where do you think that comes from?"

"Ugh, this whole time? That is creepy beyond belief," I said.

"I think it was the second or third Merlin of the White Council who engineered the whole Halloween custom. That's the real reason people started wearing masks on that night, back in the day. It was so that any hungry immortal who came by might-might-think twice before gobbling someone up. After all, they could never be sure the person behind the mask wasn't another immortal, setting them up."

"Halloween is tomorrow night," I said. A bank sign I was passing told me it was a bit after two a.m. "Or tonight, I guess, technically."

"What a coincidence," Bob said. "Happy birthday, by the way. I didn't get you anything."

Except maybe my life. "'S okay. I'm kinda birthdayed out already." I rubbed at my jaw. "So . . . if I can get to Maeve on Halloween night, I can kill her."

"Well," Bob hedged. "You can try, anyway. It's technically possible. It doesn't mean you're strong enough to do it."

"How big a window do I have? When does Halloween night end?" I asked.

"At the first natural morning birdsong," Bob replied promptly. "Songbirds, rooster, whatever. They start to sing, the night ends."

"Oh, good. A deadline." I narrowed my eyes, thinking. "Gives me a bit more than twenty-four hours, then," I muttered. "And all I have to do is find her, when she can be anywhere in the world or the Nevernever, then get her here, then beat her down, all without her escaping or killing me first. Simple."