As the Danes slumber, another sinister monster trudges toward Heorot. It is Grendel's mother, who is also dammed to spend eternity in the dark moors. She has passed the day mourning for her dead son, and she comes to Heorot seeking vengeance for his death. When she bursts into Heorot, the warriors awake and grab their weapons. She is not as strong as her son is, but she still is strong enough to devour one warrior and snatch the arm down from its place on the wall. The desire for vengeance points to "the price of slaughter/ with a loved one's life."
Hrothgar hears of the slaughter of his beloved thane Aeschere, and he hurries to the hall to mourn. Beowulf, who slept away from the hall, is summoned. Hrothgar updates him and tells him about the man that Grendel's mother killed. He also tells Beowulf that monsters like Grendel dwell in the dark moors, which are difficult to reach. Beowulf asks Hrothgar to lead him to the moors instead of mourning for his friend. Hrothgar, Beowulf, and their thanes saddle up and ride away.
At the bloodstained lake, the search party finds Aeschere's head. They also see the serpentine creatures that inhabit the murky lake, and they shake with fear. Beowulf simply calls for his armor. Unferth offers Beowulf his own sword, named Hrunting. Beowulf then announces to Hrothgar that his belongings should be sent to Hygelac if something happens. Before Hrothgar can speak, Beowulf dives into the pool.
After a long time, Beowulf reaches the bottom of the lake, where Grendel's mother waits for him. She reaches for him, but his armor protects him. He tries to cut her, but his sword can't cut her. The two begin to wrestle, but neither gains the upper hand in this combat. Beowulf spies a large sword nearby. He manages to grab it, and in one mighty blow, he beheads Grendel's mother. Light enters the murky water then. Beowulf is still angry, however, so he also beheads Grendel, who lies dead in the cave.
Meanwhile, the Danes and Geats are convinced that they will never see Beowulf againafter all, he has been underwater for such a long time. The Danes soon leave, but the Geats wait. Sure enough, Beowulf returns carrying Grendel's head and the hilt of the sword (the rest of the sword melted upon contact with Grendel's blood).
The need for repayment in some form is also a constant theme within the poem. The monsters of the poem all seek payment from life. Here Grendel's mother seeks vengeance for Grendel's death, wanting to take a life for his life. Grendel attacked Heorot because he wanted revenge for being shunned and despised. The humans think of repayment for life in monetary terms, with what is called "wergild." Beowulf is repaid for his dead man with treasures; Hrothgar cannot understand how to pay a fitting wergild to Grendel for all his lost men. The attack here is thus an attempt for Grendel's mother to retrieve the wergild on her son's life.
Hrothgar and his men show their usual cowardice in this section. Instead of asking who has killed his beloved thane and resolving to do something about it, Hrothgar merely weeps over the dead body. The Danes and Geats both quake in fear at the sight of the creatures and Aeschere's head. Beowulf, meanwhile, acts bravely, asking Hrothgar to take him to the moors, simply diving into the water instead of hanging around talking.
This battle is not as easy for Beowulf as the first one was. We knew that he could swim for great distanceswe learned this in the Breca episode. Yet it takes more than Unferth's sword to defeat Grendel's mother. In fact, the battle is won when the giant sword magically appears. This represents Beowulf's decline even in the prime of his lifefrom this point, the battles will get harder for him.
The battle can be seen as a Christian allegory. Beowulf swims to hell (the underground of the moors). It is a dark place. He does battle with the devil (Grendel's mother). Although he nearly loses, God grants him a sign that will help him win (the vision of the sword). Beowulf kills the devil, and light from heaven fills hell as a blessing. Beowulf then returns from the darkness of hell to reach the light of heaven. In this allegory, Beowulf represents Jesus' descent to hell and return to life in the Resurrection. Later the poet will compare Beowulf to Christ again.