Grendel's mother takes revenge for her son's death by killing one of Hrothgar's trusted advisors in Herot. She then takes Grendel's arm, which was hanging as a trophy in Herot, and retreats back to her underwater lair.
There is many versions but in Beowulf: A New Telling by Robert Nye she takes Aschere's body, Unferth, and Grendel's arm.
to avenge her son and take his arm
Beowulf and Grendel's mother fought in her underwater lair at the bottom of a dark, eerie mere (a kind of lake). The battle took place in the depths of the water, adding to the suspense and danger of the encounter.
After Grendel's mother attacks Herot and kills one of the warriors, she takes Grendel's arm as revenge for his death.
Herot
A swamp
Beowulf followed Grendel's mother's tracks to her underwater lair after she attacked Heorot. He dove into the lake and ultimately battled and killed her with the help of a magical sword he found in her lair.
he dies later on
Underwater, in a lake.
Under water, where Grendel's mother lived.
Grendel's trip to Herot is considered miserable because it is a place of light, warmth, and beauty, contrasting with Grendel's dark, cold, and lonely existence. The noise and revelry of Herot emphasize his isolation and exclusion from human society, making him feel even more alienated and miserable.
Grendel's mother
The Sword breaks
To steal back Grendels arm