In the above statement who is cain abel? what is their relationship to grendel

Cain and Abel are the first sons, according to the bible to be born from Adam and Eve. The writer of Beowulf, is using a biblical allusion. (A reference to another literary work). In the Bible Cain committed the first murder by killing his brother Abel. The book calls Grendel "kin of Cain" because of Grendel's sinful and murderous ways. 

The Things They Carried