The name Nimue is a clerical error for Nineve or Nyunyue, and the character is named Viviane in the Old French "Merlin". Some sources make her an entirely separate character, saying that she was the daughter of a Sicilian siren. For the sake of convenience, I have adopted the name "Nineve" for Malory's first British Lady of the Lake, introduced and disposed of before Nimue's appearance.
No force, said Merlin, hereby is a sword that shall be yours, an I may. So they rode till they came to a lake, the which was a fair water and broad, and in the midst of the lake ... an arm clothed in white samite, that held a fair sword in that hand. Lo! said Merlin, yonder is that sword that I spake of. With that they saw a damosel going upon the lake. What damosel is that? said Arthur. That is the Lady of the Lake, said Merlin; and within that lake is a rock, and therein is a fair place as any on earth, and richly beseen; and this damosel will come to you anon, and then speak ye fair to heer that she will give you that sword.
The damsel greeted Arthur and agreed to give him the sword Excalibur if he would give her a gift when she asked it. When he promised to do so, she pointed out a barge in which he could row over and take the sword and scabbard.
came on horseback, richly beseen, and saluted King Arthur, and there asked him a gift that he promised her when she gave him the sword ... Well, said the lady, I ask the head of the knight that hath won the sword, or else the damosel's head that brought it; I take no force though I have both their heads, for he slew my brother, a good knight and a true, and that gentlewoman was causer of my father's death. Truly, said King Arthur, I may not grant neither of their heads with my worship, therefore ask what ye will else ... I will ask none other thing, said the lady. When Balin ... saw the Lady of the Lake, that by her means had slain Balin's mother, and had sought her three years; and when it was told him that she asked his head of King Arthur, he went to her straight and said, Evil be you found; ye would have my head, and therefore you shall lose yours, and with his sword lightly he smote off her head before King Arthur.
Arthur was greatly displeasured and banished Balin from court, Balin still maintaining that
this same lady was the untruest lady living, and by enchantment and sorcery she hath been the destroyer of many good knights, and she was causer that my mother was burnt, through her falsehood and treachery.
Arthur buried the lady richly, but we may probably accept Balin's accusation as accurate. This first British Lady of the Lake, if not one of Merlin's cohorts, was at least willing to work in concord with him; yet when Merlin returned to court and learned of what had happened, Malory does not record that even he made any attempt to deny Balin's charges.