“Signs in the sky” possibly refers to Jesus’ who said his return and the end of days will be signaled by signs in the sky. It could just be a generalisation and not refer to anything specific. The star of Bethlehem was a sign in the sky. Comets are supposed to be portents of doom. The demon could be referring to superstition in general. I’d have to re-read the passage to be more certain but I don’t have a copy of the book anywhere near me.
Demons were thought by ancient cultures to inhabit a void (empty space) belonging neither to this physical plain of reality nor heaven or hell. It is a term often used to refer to the heavens (outer space) but I suspect not in this instance.