An allusion is a reference to a well-known person, place, or thing that exists outside of the literary work. A biblical allusion is simply a reference to a well-known person, place, or thing from the Bible.
Here, the story of Cain and Abel (a biblical story that everyone would recognize) is being referenced. Here, an allusion to Cain is made to explain the origin of Grendel.
The passage that contains a biblical allusion is in letter C. On kin of Cais was the killing avenged, by sovran God for slaugthered Abel, ill fared his fued, and far was he drive, for the slaughter's sake, from sight of men. Of Cain awoke all that woful breed. I hope this would help you. Have a nice day!
