Duel between Merlin and Nimueh

A magical duel between Merlin and Nimueh occured in the Isle of the Blessed. When Arthur Pendragon got bitten by a Questing Beast, Merlin was forced to go to the Isle of the Blessed where he met Nimueh again. Merlin offered to give his life to save Arthur's life, Nimueh instead took his mothers life while Arthur was saved. When this happened Gaius visited Nimueh and gave his life, saving Merlin's mother

Merlin encountered Nimueh again and they started duelling with their magical powers. Nimueh apparently emerged victorious when Merlin seemed to be killed but he later recovered. The duel ended when Merlin killed Nimueh before reviving Gaius. (Le Morte d'Arthur)