This one is just bloody terrifying full stop. Monsters hiding in closets or under the bed are a thing of nightmares for most kids. A skeletal type monster with crazy hair would definitely terrify me as child.
This image is perhaps almost to dark in its representation, although there is certainly a dark thread running through the novel. However this simplistic representation of smoke and mirrors might be better aimed at the more mature spectrum of a YA readership. However that said I think the blue smoke could work well with the castle turrets perhaps edging out of the top. with the text layers then on top and centered.
stunning portraiture. The other notable is that we are not used to thinking of sculptures as such, if they also combine paint. This is mostly because in the classical greek/roman sculptures, the paint has worn off exposing the material. Not because there wasnt paint to begin with.