‘She has taken up the feud
Because of last night, when you killed Grendel,
Wrestled and racked him in ruinous combat
Since for too long he had terrorized us
With his depredations. He died in battle,
Paid with his life; and now this powerful
Other one arrives, this force for evil
Driven to avenge her kinsman’s death.
Or so it seems to thanes in their grief,
In the anguish every thane endures
At the loss of a ring-giver, now that the hand
That bestowed so richly has been stilled in death.’

Beowulf, Seamus Heaney (trans.)

Lines 1333-1344: Hrothgar says that Grendel’s mother has set out to avenge her son’s death by killing someone from the tribe that killed him.