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.
“We do not grow absolutely, chronologically. We grow sometimes in one dimension, and not in another; unevenly. We grow partially. We are relative. We are mature in one realm, childish in another. The past, present, and future mingle and pull us backward, forward, or fix us in the present. We are made up of layers, cells, constellations.” Anais Nin