"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."

"Yep. Almost impossible, but simple. And at least you know the when and the how," Bob said.

"But I'm no closer to why."

"Can't help you there, boss," the skull said. "I'm a spirit of intellect, and the premise we're dealing with makes no sense."

"Why not?"

"Because there's no reason for it," Bob said, his tone unhappy. "I mean, when Maeve dies, there will just be another Maeve."

I frowned. "What do you mean?"

Bob sighed. "You keep thinking of the Faerie Queens as specific individuals, Harry," Bob said. "But they aren't individuals. They're mantles of power, roles, positions. The person in them is basically an interchangeable part."

"What, like being the Winter Knight is?"

"Exactly like that," Bob said. "When you killed Slate, the power, the mantle, just transferred over to you. It's the same for the Queens of Faerie. Maeve wears the mantle of the Winter Lady. Kill her, and you'll just get a new Winter Lady."

"Maybe that's what Mab wants," I said.

"Doesn't track," Bob said.

"Why not?" I asked.

"Because the mantle changes whoever wears it."

My guts felt suddenly cold.

(I'm not Lloyd Slate.)

(Neither was he. Not at first.)

"Doesn't matter who it is," Bob prattled on. "Over time, it changes them. Somewhere down the line, you wouldn't be able to find much difference between Maeve and her successor. Meet the new Maeve. Same as the old Maeve."

I swallowed. "So . . . so Lily, who took the Summer Lady's mantle after I killed Aurora . . ."

"It's been what? Ten years or so? She's gone by now, or getting there," Bob said. "Give it another decade or two, tops, and she might as well be Aurora."

I was quiet for a moment. Then I asked, "Is that going to happen to me, too?"

Bob hedged. "You've . . . probably felt it starting. Um, strong impulses. Intense emotions. That kind of thing. It builds. And it doesn't stop." He managed to give the impression of a wince. "Sorry, boss."