Kenning is a figurative expression or metaphor used to point or name something instead of it proper noun. The Kennings were used extensively in Anglo-Saxon poetry. Beowulf is full of them, here are some examples which were used for Grendel and his mother:
Shepherd of Evil (Grendel)
shadow of death (Grendel)
Mankind enemy (Grendel)
She-wolf (Grendel's mother)
Mighty Water Witch (Grendel's mother)
All of them tells that the poet looks at both of them as symbol of evil but along with that source of mighty power also.
