Grendel's mother (sometimes called his "dam") is not as huge or as powerful as the son, but she is motivated by revenge. Her son has returned to their cave mortally wounded, one of his two arms (or claws) ripped from its shoulder socket and hanging, now, beneath the roof of Hrothgar's mead-hall. Instead of cowering in grief, the mother seeks revenge.
Although the Danes have heard that the swamp may harbor two ogres, they seem to believe that the problem is solved when Beowulf defeats Grendel. On the night after that victory, the Scyldings celebrate with a great deal of food and drink. Many of the celebrants spend that night in Heorot while Beowulf sleeps elsewhere. The mother stalks up from her mere, retrieving her son's claw and murderously abducting one of the Scyldings from the mead-hall.
When Beowulf comes after her, the mother has another advantage. She is in her home territory, which she has ruled for a hundred years. As the Geat champion dives deep into the lake, the mother waits and attacks only when he nears the bottom. He is virtually helpless as she drags him to the dry, eerily lighted cave for the kill. Once on dry land, however, Beowulf is able to mount a counter-attack. Although his sword, Hrunting, loaned to him by Unferth, fails to penetrate the mother's hide, Beowulf discovers a giant magic sword in the cave and is able to kill the mother with it. The sword melts to its hilt after Beowulf uses it to decapitate the corpse of Grendel, which lies nearby. He returns to Heorot with a greater trophy, the head of the ogre, as well as the hilt of the magic sword.
Some critics feel that Grendel's mother receives inadequate consideration in the poem. Her motive is as human as it is monstrous as she seeks revenge for her defeated son and reclaims his arm, which from her point of view must seem a barbaric trophy. She has lived in the mere for a hundred years and was never the problem that her son was. Nevertheless, this is Beowulf's poem; the mother is just another monster in a heroic epic. Other writers will have to enhance her tale.