Grendel's mother's lake is in a dark, rocky area. On the cliff overlooking the water they find Aeschere's head. The water below, filled with serpents, boils with blood. Sea monsters rest on the shore.
The monsters' status as outcasts from society is symbolized by the desolation of their home.
Beowulf, completely without fear of death, puts on his armor and grasps his weapons. Unferth lends Beowulf Hrunting, a sword that has never failed and has been passed down in Unferth's family.
Unferth again speaks with Beowulf, but this time he is generous instead of jealous, much to his credit as a man and warrior.
Beowulf asks Hrothgar to protect his Geat companions and send the treasure he's won to Hygelac, should he fail to return from the fight with Grendel's mother. He then gives his own sword to Unferth, and plunges into the water.
Beowulf shows loyalty to his king, a sign of a good warrior, and to his men, a sign of a good king.
Beowulf has to swim for "part of a day" before he reaches lake bottom. When he lands, Grendel's mother grabs and squeezes him, but his armor protects him. She carries him to her "hall," a cave protected from the water.
Grendel's Mother's "hall" is an underwater cave, again emphasizing her outcast status. Her "hall" is inaccessible to society.
Beowulf strikes at Grendel's mother with the borrowed sword Hrunting, but the blade has no effect and actually breaks. Unfazed, Beowulf grabs the monster by her hair and pulls her to the floor, attempting to defeat her with his bare hands. She meets him blow for blow.
Like Grendel, Grendel's mother is immune to normal weapons. Beowulf again shows courage and is rewarded for it by fighting with his bare hands, as monsters do.
Grendel's mother stabs Beowulf with a knife, but his mail shirt blocks the blow. Beowulf then notices, lying among the armor on the cave floor, an old sword made by giants so large few could use it. He grabs the sword and strikes Grendel's mother, slicing through her neck. She falls to the floor, dead. The cave blazes with light.
Beowulf's courage seems to influence fate by leading to his discovery of the giant sword. The sudden burst of light at Grendel's Mother's death may be a sign from God, but it's left unexplained.
In a corner, Beowulf sees Grendel's lifeless body. Still in fury at Grendel's awful deeds, he cuts off Grendel's head.
Just as Grendel's mother took Aeschere's head, Beowulf takes Grendel's.
Hrothgar and the other Danes, who have been waiting on the shore, see blood bubble to the surface of the lake. They think that Beowulf has been defeated and leave the lake in great misery. But Beowulf's Geats remain behind.
The Geats, as Beowulf's kinsmen, show him greater loyalty.
Back in the underwater cave, Grendel's blood melts the giant sword until only the hilt remains. Beowulf swims back to the surface with the sword hilt and Grendel's head. When he surfaces, the Geats are joyful. Four of them carry Grendel's head on their spears, and they return to Heorot.
Beowulf takes the sword and head because they confirm his victory and therefore ensure his fame. Beowulf must be a powerful warrior: it takes four men just to carry Grendel's head!