Q - I have to agree that the Erlking was in danger.
The DH was going to be performed on Halloween. The Erlking is an immortal with a Mantle/Mask. We know they are vulnerable on Halloween. I don't see why he'd be immune.
Wait, I see where we are talking Apples to Oranges here. The Darkhallow was Eating Mortal Ghosts; that is what was fueling the Spell itself and what the Erlking was summoned to stir up. The Erlking was not part of
that. However, he'd still be a being who's machinery runs on Life Energy, so he'd have to book it fast into the NN to avoid the Desolation Vacuum of Life that would immediately follow.
Since the Vacuum is required for the Spell to work (because that's how Vacuum works) then it might just sound like semantics, but I see it as opposite sides of the Energy Balance. Had the Erlking been in danger of being Eaten as part of the initial Pull-down stage then a)the Vacuum created would increase, and B) the necrogod that popped out the other side would have
Absorbed an Immortal Mantle /already/ rather than having Created a whole new one from the mortal Spirits as described.
So he might well be in danger of Dying from the Fallout. I hadnt considered that too hard, I was discounting on the assumption that can pop in and out of the NN like Toot&Co and would have enough reaction-time to do so (which is not guaranteed by any means I admit). But by all accounts the Darkhallow creates a New Mantle, it doenst need an existing one as a foundation or anything. And if the Elrking did die from teh Fallout, his Mantle would pass to It's own Next in Line (a new Hunter? the Next Eldest of the Goblins?) rather than being subsumed into the Necrogod; In other words Erl was in Danger but his Mantle was not.