In Beowulf, we first encounter her after Grendel’s death which she avenges by entering Heorot and killing Æschere. Then Hrothgar tells Beowulf about her.
In Kevin Crossley-Holland’s translation, the passage reads as follows: “The wandering, murderous monster slew him/ in Heorot; and I do not know where that ghoul,/ drooling at her feast of flesh and blood,/ made off afterwards. She has avenged her son….now another mighty/ evil ravager has come to avenge her kinsman….I have heard my people say,/ men of this country, counsellors in the hall,/ that they have seen two such beings,/ equally monstrous,/ rangers of the fell-country,/ rulers of the moors; and these men assert/ that so far as they can see one bears/ a likeness to a woman….These two live/ in a little-known country, wolf-slopes, windswept headlands,/ perilous paths across the boggy moors, where a mountain stream/ plunges under mist-covered cliffs,/ rushes through a fissure” (84-85).
Beowulf swims all day to get to her home underneath a snake-infested lake. They fight, his sword is powerless against her. His corselet protects him when she is about to stab him. Then he sees a sword made by giants; this enables him to behead her. He beheads the already dead Grendel with that sword, whose blade melts after encountering the monster’s blood. Beowulf returns with Grendel’s head and everyone celebrates.