If the story is primarily about Beowulf, the hero, why do you think the story opens with a long narrative or discussion of Grendel?

do you have more to the question?

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.

i think the literacy device is "it's frozen head"