Definition:
Ominous - threatening, suggestive that something bad will happen or be revealed.


Example:
“At length Frodo, groping along the left-hand wall, came suddenly to a void. Almost he fell sideways into the emptiness. Here was some opening in the rock far wider than any they had yet passed; and out of it came a reek so foul, and a sense of lurking malice so intense, that Frodo reeled.”
~ J. R. R. Tolkien, The Two Towers


Explanation:
This passage is filled with a sense of foreboding. Tolkien uses words that bring up feelings of fear in the reader and lets them know that something terrifying is about to happen to the character of Frodo. The reader knows that the tone is ominous through this word choice, and is prepared for what is to come - unlike poor Frodo.