Grendel, Who was mortally wounded, fled "to his den,/ to [h]is miserable hole at the bottom of the marsh"(394-395).
In retaliation of her sons' death, Grendel's mother comes in the night and "kills Hrothgar's closest friend and carries off her child's/ claw"(Raffel, 51).
Upon hearing about the mother's attack, Beowulf went to the entrance to her domain and "leaped into the lake," against everyone's pleas (468).
As soon as Beowulf entered the mothers' domain, however, she "welcomed him in her claws," and brought him "into someone's battle hall" (475, 486).
In this hall, Beowulf was forced to fight Grendel's mother. He quickly realized, however, that "no sword could slice her evil / [s]kin" (496-97). No sword, but the very one she kept as a trophy!
After Beowulf slew Grendel's mother, he returned with his two trophies. The only treasures he left the cave with, were "Grendel's head and the hilt of the [. . .]/ sword" that he had slain the mother with (587-588).