Here's why:
* Metaphor: A figure of speech that compares two unlike things without using "like" or "as." In this case, the shadows are being compared to something that is driving, which is a human action, not something shadows do literally.
Let's break down the phrase:
* Driving back: This suggests an active force pushing something back.
* Shadows: This refers to darkness or the absence of light.
* Louring hills: This creates a sense of ominousness and suggests the hills are dark or forbidding.
Therefore, the phrase creates an image of something (perhaps the sun or daylight) actively pushing back the darkness over the ominous hills. This metaphorical language creates a vivid picture in the reader's mind and evokes a sense of drama or conflict.