Grendel's mother is feared because she is a parent, griefstricken and angry at the death of her child. Unlike Grendel, his mother is able to use magicto aid in the defeat of her enemies. In order to do battle with Grendel's mother, Beowulf must go to her lair under the lake and find a way to overcome her because she has enchanted his sword, making it dull and powerless. Beowulf was nearly defeated when he discovered an unenchanted sword made by giants and he used that sword to decapitate her.
Grendel is a fierce animal, a monster of surpassing strength. However, his mother possesses both an intelligence he lacks, and supernatural abilities. She lives at the bottom of a lake and can control both the water and the animals. People fear her because she is a cunning and intelligent witch with powers they don't understand.
Beowulf is unable to kill her with his own sword. She is too protected by her magic. However, he spies in her lair the sword that was made giants, and kills her with a slice to the throat.