"The Interlopers" by H.H. Munro (Saki) is a short story which is more about plot than anything else; however, it also contains some elements of figurative language, as you suggest.
Idioms are the peculiarities of speech that are distinctive to a speaker or the characters of a region. Georg and Ulrich both talk in a rather formal manner, without a lot of contractions or abbreviated words which are the easiest idioms to find; however, they do use some terminology which could be considered colloquial (specific to their region). I have highlighted them in bold below. George says:
"How the whole region would stare and gabble if we rode into the market-square together. No one living can remember seeing a Znaeym and a von Gradwitz talking to one another in friendship. And what peace there would be among the forester folk if we ended our feud to-night. And if we choose to make peace among our people there is none other to interfere, no interlopers from outside ... You would come and keep the Sylvester night beneath my roof, and I would come and feast on some high day at your castle...."
Imagery is the use of descriptive language to create an impression for the reader. In terms of imagery in this story, the author draws a vivid picture, though again the emphasis is more on plot than consistent artful description. Here are a few examples of imagery:
- "as boys they had thirsted for one another's blood"
- "peering through the tree trunks and listening through the whistling and skirling of the wind and the restless beating of the branches for sight and sound of the marauders"
- "a deed of Nature's own violence overwhelmed them both."
The author does not use a lot of similes and metaphors, but here is one vivid example from early in the story:
- "The roebuck, which usually kept in the sheltered hollows during a storm-wind, were running like driven things to-night...."
This particular simile also serves as a foreshadowing of things to come, as it is an unnatural occurrence which is happening in the forest on this stormy night.
Most of the examples of personification in this story center around the trees of the forest, which is not surprising since that is where most of the action in the story takes place. Consider the following (bold print is mine):
- "a mass of falling beech tree had thundered down on them"
- "The descending twig had slashed the skin of his face."
In fact, the entire concept of the trees "trapping" the two men is personification.
Again, this story is centered around plot, but there are certainly some examples of figurative language in this story.