The Lady of the Lake was an otherworldly woman who makes many appearances throughout the Arthurian cycle. There are two distinct Ladies of the Lake in Malory and seems to be three in total when the Vulgate is taken into account.
   One, the Dame du Lac (Vivienne), raised Lancelot in France, the Lady Lile of Avelion gives Arthur Excalibur (Malory, Book 1 Chapter 25) and is later killed by Balin (Malory, Book II, Chapter 1) and finally Nimue, the "temptress" who beguiled Merlin and succeeded in shutting him up under a stone or in a tree.
   In the legends, Viviane definitely and Nimue apparently had other Damsels of the Lake under them and the Vulgate states that they owed their knowledge of magic to Merlin. This has led to speculation that behind the Ladies of the Lake motif lies a memory of an ancient pagan sisterhood. There is also the possibility that behind the legendary figure of the Lady of the Lake we have echoes of the ancient Goddess who can also be equated with the idea of Sovereignty.

The first Lady of the Lake in Malory
Nimue

Dragon's Head - Daughters of the Dragon
A new look at Merlin and the Lady of the Lake legends