Answer:
Metaphor
Explanation:
A metaphor has a structure of saying something "is" something, even though what it means is that something "is like" something else in terms of similar characteristics.
In this case, the hills "were" an angry dragon's backbone. That does not mean that the hills were literally an angry dragon's backbone, but it is like one, probably to show the effect that the hills were restless and dangerous, just like an angry dragon's backbone.
