Cain’s clan, whom the Creator had outlawed
and condemned as outcasts. For the killing of Abel
the Eternal Lord had exacted a price:
Cain got no good from committing that murder
because the Almighty made him anathema
and out of the curse of his exile there sprang
ogres and elves and evil phantoms
and the giants too who strove with god
time and again until He gave them their reward.-Beowulf, Seamus Heaney translation, l.106-114
So we take monsters, but give them an Old Testament back story. This sort of mixing of folk beliefs with the Biblical narrative reminds me of Lewis’s “redeemed paganism”, featured throughout the Narnia Chronicles. It’s no stretch at all to say he is doing the same thing the Beowulf author was doing with his story.