Grendel's Mother (also sometimes called "Hag" in the old Viking Beowulf saga) is the main antagonist in the Viking saga Beowulf as well as its many retellings over the years: she has varied between a hideous old hag to a seductive femme fatale (similiar to a siren) but has always remained consistant in her motives - to avenge the death of her son Grendel at the hands of the Viking hero Beowulf.
Grendel's Mother has always been associated with the water and may well of been based on an ancient Slavic water-goddess or spirit: at any rate the monster attacked Beowulf and attempted to drag him to a watery grave (at least in the originally telling of the story) but was defeated by Beowulf in an epic struggle.
However in the modern retelling of Beowulf made famous by the Rendered 3D film, Grendel's Mother was not killed by Beowulf. When he entered her cave, she took the form of a beautiful nude woman with gold skin. She was impressed by his strength and courage and saw in him the savagery of a beast and the glamour of a king. She swore to make him a king if he would mate with her and give her a son to replace Grendel. She seduced him and they conceived a child together. Their offspring was a perfect gold-skinned male who took the form of a mighty dragon. She and their son went on to torment him for a great portion of his life. This is inconsistent with the old Norse saga - in which Beowulf slew her only to die years later at the hands of a dragon.
She is portrayed by Angelina Jolie.