Describe the encounter between Beowulf and Grendel's mother.

Following the death of her son, Grendel's mother attacks Herot, kills Esher, and flees with her son's claw. Beowulf follows the monster to the entrance of her lair, which is at the bottom of a dangerous, monster-infested lake. He proceeds to put on woven mail armor to protect him from Grendel's mother's claws and leaps into the lake with Unferth's sword named Hrunting. Grendel's mother then grabs onto Beowulf and drags him into her underwater lair, which is actually a battle hall.
Beowulf initially strikes Grendel's mother's head with Hrunting, which does not harm the monster at all. He then throws away his ineffective sword and proceeds to fight Grendel's mother with his bare hands. The two engage in a long struggle, and Beowulf begins to tire. When Beowulf stumbles, Grendel's mother manages to stab him with a knife, but his chain-mail saves his life.
Beowulf then spots a magical ancient sword from the days of the giants on the wall of the battle hall and uses it to kill Grendel's mother by striking her in the neck with the sword. After Beowulf kills Grendel's mother, he proceeds to decapitate her and swims to the surface of the lake.