Grendel's mother, monstrous hell-bride, brooded on her wrongs. She had been forced down into fearful waters, the cold depths, after Cain had killed his father's son, felled his own brother with a sword.

Grendel’s mother,
monstrous hell-bride, brooded on her wrongs.
She had been forced down into fearful waters,
the cold depths, after Cain had killed
his father’s son, felled his own
brother with a sword.

– Beowulf, Seamus Heaney (trans.)

Lines 1258-1263: So even the monster Grendel has a mother. And not surprisingly, she is depicted as being so monstrous and evil that she is put on the level of the bride of hell. She comes back to reak havoc in Heorot after Grendel is killed by Beowulf.

Beowulf Quotes