Onomatopoeia refers to all those words that imitate, resembles, or suggests the natural sounds they describe. "Clash" is a perfect example of onomatopoeia because it imitates the sound of a clash or a violent confrontation. This word also contributes to the overall mood of the lines: It evokes an emotion of dread.
Some other examples of onomatopoeia are "clang" and "roar" (Written in the first line) "smash", "clap", "pop", "bang" "cough" and "moo".
