According to Malory, she was a great sorceress who fell in love with Arthur and entinced him to her tower in the Forest Perilous (Perilous Forest). When she could not get him to make love to her, she plotted his death.
Then every day she would make him ride into that forest [where his own knights were looking for him], to the intent to have King Arthur slain. For when this Lady Annowre saw that she might not have him at her will, then she laboured by false means to have destroyed King Arthur.
Nimue learned of the situation and came into the same forest, seeking Lancelot or Tristram to help the King. She found Tristram and brought him to the tower in time to see two strange knights defeating Arthur and unclacing his helm.
And the Lady Annowre gat King Arthur's sword in her hand to have stricken off his head.
Tristram rushed up shouting,
Traitress, traitress, leave that
and killed the two knights. Meanwhile, Nimue shouted to Arthur not to let Annowre escape, so the King chased Annowre and smote off her head, which Nimue hung up by the hair to her saddlebow.
She is called Elergia in La Tavola Ritonda.
Le Morte Darthur | Sir Thomas Malory, 1469-1470