Jeez people, I know Jim likes to play tricksy Sidhe "gotcha" games, but sometimes we just have to take the words for what they actually say.
Jim
explicitly says that the more lives that would have been sacrificed, the more power the caster would receive. All the life energy not protected by necrotic magic would have been consumed. All the ghosts would have been consumed, all the lives of all the people, the flowers, and bugs, in the city would have been consumed, and if the Erlking was around, he would have been consumed too, along with all his power (which by itself would have been enough to make an new Immortal). The Erlking wasn't consumed by the Darkhallow because the Darkhallow was interrupted before it could go off (hence Chicago not being a giant cemetery in the books). Maeve's mantle didn't get destroyed when Maeve died because that mantle is it's own magical construct, a discrete thing that exists beyond any one person, and there wasn't any necro-vortex trying to eat it at the moment.
We
know that Halloween is when Immortals grab power from each other. That's part of the whole Dresdenverse origin of "Halloween" as we know it. On Halloween, the Erlking was mutable and could have his power partially or wholly consumed, or even be killed. If he has a mantle, as in a magical construct that sits on top of him like the WK mantle sits on Harry, then even that would have been wholly consumed added to the new Immortal's power. That's the whoooooole point of all this "mutable on Halloween" business. If it
wasn't like that, then there'd be virtually no point in adding in that particular detail,
especially in bringing up the Darkhallow specifically. Jim could have and probably would have left it at "Immortals can die on Halloween".
“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'm just not sure how much clearer it can get without Jim himself coming in a laying it out bare one way or another. Maybe someone can ask him at the next Q&A he does.
This is something that's been tossed around for years now. People are freaking
obsessed with mantles. Everything's a mantle now, everybody's got a mantle.
Not every "mantle" is a discrete magical construct like the Queens and Knight have that hops from one vessel to the next. Immortals have a mantle of immortality that's it, I don't see anything that says it *has* to zip onto someone new, the Queens' mantles do that because it's part of what they are, they're cosmically important.
Sometimes the "mantle" is just like, an idea, basically, like being the "leader".
Being a leader doesn't give you an explicit personal power boost like you can all of a sudden lift heavy objects over your head. What it does do is give you power, because other people might follow your orders when you tell them to help you lift heavy objects. When a leader is gone, sometimes someone takes up the role, sometimes the group itself is dismantled (pun totally intended). There seems to be a mighty fine line between things that are just ideas in people's heads, and things that are actually magic, I recognize that, but the line is there.
Erlking is an immortal, he has the "mantle" of immortality, but there's nothing that says he has a "mantle" like Vadderung has Santa, or that it's something that will get passed along when he dies. His power might just die along with him and disperse like a body turns into dust, ready to be used in creating something new. Nothing says that when Vadderung stops being Santa, that the mantle goes to someone else, it might not even exist outside the realm of ideas.