Final answer:
The lines contain a good example of alliteration, a rhetorical device used by Anglo Saxons.
Step-by-step explanation:
The lines "A powerful monster, living down / In the darkness, growled in pain, impatient / As day after day the music rang / Loud....." contains a good example of the rhetorical device used by Anglo Saxons known as alliteration. Alliteration is the repetition of consonant sounds at the beginning of words in close proximity. In this case, the repeated 'd' and 'p' sounds in the words 'down,' 'darkness,' 'growled,' 'pain,' 'impatient,' and 'day' create a rhythmic and musical effect in the poem.
Learn more about alliteration