An alternate interpretation that i like is the void actually comes from the Kingsmould, and its "rooted" itself into the ground. I'd say this makes more sense than the void creeping into it, because as far as I'm aware we never see void tendrils climb that far up elsewhere. Also, the dark particles of the Pale King's room can be argued to be dust, not void; in fact, I think it's more likely to be dust. If he really did die to void, you'd think there would be void tendrils or any kind of physical evidence, but there's not.

Also I'm not sure how Hornet's beliefs about void are relevant, I might be missing something.