What "ancient feud" was mentioned in ch.4? How does it relate to grendel

Grendel is wandering the dark forest and hears the Shaper's song. The Shaper sings about an ancient feud between two brothers. Of course this is a Biblical allusion to the Old Testament story of Cain and Abel. In the story, Cain kills Abel out of jealousy. God had split the world in two, the cursed (Cain) and the blessed (Abel). The Shaper claims that Grendel is on the side cursed by God; Grendel is a descendant of Cain.