Idiom

When I read the definition of idiom:

The term refers to a set expression or a phrase comprising two or more words. An interesting fact regarding the device is that the expression is not interpreted literally. The phrase is understood as to mean something quite different from what individual words of the phrase would imply.

I was immediately reminded of the metaphor:

Metaphor is a figure of speech which makes an implicit, implied or hidden comparison between two things or objects that are poles apart from each other but have some characteristics common between them.

Further research revealed that the two concepts may overlap but are distinct.

• Difference Between Metaphor and Idiom
• Idioms vs. metaphors?

The metaphor is a comparison between two things to help the reader understand the relationship. The idiom is an expression that has a hidden meaning that is meant to be interpreted differently than the literal meaning of that same expression.

Idioms in Fantasy-Fiction Writing
When writing your story, remember to keep the language consistent to your character(s) and setting. The idioms in your writing will be more natural and might not be perceived as cliche.

I plan on illustrating this by finding examples in published fantasy-fiction.

Learn More About the Topic
Definition
Anachronistic Idioms In Fantasy Writing
Idioms and Catchphrases
Idioms and clichés in creative writing
Using Idioms In Fantasy and Science Fiction
Using Idioms to Add Flavor to Your Writing