Final answer:
Grendel's statement about Hrothgar's warriors becoming hornets is an example of a simile.
Step-by-step explanation:
The literary device being used when Grendel says Hrothgar's 'warriors became hornets' is simile. A simile is a figure of speech that compares two things using the words 'like' or 'as'. In this case, Grendel is comparing Hrothgar's warriors to hornets, emphasizing their ferocity and aggressiveness in battle. The simile helps to create a vivid image in the reader's mind, highlighting the warriors' transformation into powerful and formidable adversaries.