"High above this quagmire of violence rise the sunny plateaus of Eden, casting their shadows before."
The wording is meant deliberately ambiguous.
It opens a paragraph about a group of people that have retreated from the world but are about to attempt to conquer it (metaphorical foreshadowing).
At the same time, the inhabited plateaus, similarly to the Greek Meteora monasteries, cast a literal shadow onto lower places. The reader is taken to the peaceful, heavenly place and its inhabitants, before pointing out their conquest plans.
The aspects of the question are
Does the average reader recognize the hint at the familiar metaphor, even if only half the original expression is used (omitting the "coming events")? Does the reader only associate physical shadows with that wording ... or does it just sound wrong?
Does the metaphor always have an evil connotation (cf. 3rd metaphorical example below) or can it just mean that something big is about to happen? I am trying to paint the picture of a peaceful, lush place at the beginning of the paragraph, slowly grading into the darker aspects.
How could it be more properly worded?
I have found three examples of metaphorical usages so far (thanks to Kate Bunting for the first one):
coming events cast their shadows before
Clues indicate important events to follow.
cast a long shadow
To continue to have consequences well into the future.
A: "I know I made a mistake, but that happened years ago! Why are we still talking about it?" B: "Because old sins cast a long shadow."
[However I don't know how reliable this source is, as the Cambridge Dictionary doesn't mention this meaning]
cast a shadow over/on sth
to spoil a good situation with something unpleasant:
Her father's illness had cast a shadow over the birth of her baby.