Beowulf heard about Grendel's mother seeking revenge for her dead son, Grendel. He decides to kills her and goes to her lair "where water pours / [f]rom the rocks, then runs underground" (Raffel 426-427).
He swam for hours until she discovered him. she "carried him, armor / [a]nd sword and all, to her home" (479-480).
While fighting Beowulf "[d]iscover[es] that no sword could slice her evil skin" (496-497). He drops his sword and tries to fight her using his hands which doesn't last long.
Beowulf was in his last moments when he saw a "[s]word, hammered by giants, strong / [a]nd blessed with their magic, the best of all weapons"(514-515).
Beowulf picks up the sword and "struck with all the strength he had left / [h]er body fell / [t]o the floor, lifeless"(521, 523-524).
After killing her, Beowulf goes into her house and finds Grendel's body.He takes his sword to get revenge for all the people grendel killed by cutting "[h]is head with a single swift blow" (544).