The phrase "Horrendous horrors haunted Helen's happiness" illustrates alliteration. Alliteration is the repetition of the same consonant sound at the beginning of two or more words in close proximity, such as the "h" sound in this phrase. Alliteration is often used in poetry and prose to create a musical or rhythmic effect, to emphasize certain words or ideas, or simply for aesthetic purposes. In this example, the use of alliteration also adds to the ominous and unsettling tone of the phrase, emphasizing the negative impact of the "horrendous horrors" on Helen's happiness.