In Grendel, what poetic device is used in “ragged little bands that roamed the forest on foot"?

Grendel, a novel by John Gardner, gives us the classic story of the epic poem Beowulf from the monster Grendel's perspective. While Grendel is not written in poetic verse as Beowulf is, it still makes great use of literary and poetic devices. 
The specific quote you are asking about uses the poetic device of alliteration twice. Alliteration is the repetition of letters/sounds at the beginning of words that are consecutive or close to each other in a sentence or phrase. In your quote, we find repetition of the letter "r" with the words "ragged" and "roamed," as well as repetition of the letter "f" with the words "forest" and "foot." If you move a little further in the sentence your phrase is from, to "crafty witted killers that worked in teams," you will see even more alliteration with "crafty" and "killer" and "witted" and "worked."