The Fate of Merlin
There were several versions of Merlin's death or his mysterious disappearances from the legend. As I said before, Merlin disappeared before Arthur was born in the works of Geoffrey of Monmouth (Historia regum Britanniae, 1137) and Wace (Roman de Brut, 1155). Merlin was involved in Arthur's conception, but doesn't appear again in Arthur's reign.
In most later tales, Merlin was still alive when Arthur became king. In Didot Perceval, he outlived Arthur and the Knights of the Round Table, in the final battle against Mordred. It was he who guided Perceval in the final stage of the quest for the Holy Grail. Merlin told Perceval that he would not die until the end of the world.
In a few tales, a maiden or a fay had trapped Merlin in an enchantment. The most famous was the Lady of the Lake. Again, there are few versions involving his death at the hand of the Lady of the Lake.
The Lady of the Lake was a powerful sorceress and the lady of Otherworldly realm, hidden by the illusionary lake. The Lady of the Lake was known by several other names, such as Niniane, Viviane and Nimue. Further confusion resulted when some author listed several women with title the Lady of the Lake.
The variation of names depends on the authors, but whatever her name was, the most important one was foster-mother of Lancelot and sorceress who had trapped Merlin in a enchantment. She appeared as either Niniane or Viviane in the Vulgate or Post-Vulgate cycles; while in Morte d'Arthur, Malory called her Nimue.
In Vulgate Merlin, Niniane or Viviane, the Lady of the Lake had first met Merlin, when she was only twelve. She was amazed by the power of Merlin. She promised to love him if Merlin would teach her all his crafts. Years later, Merlin met Niniane again. Through subterfuge, Niniane seduced and used her magic to confine in a enchanted tower in which Merlin was powerless to leave, while the Lady could visit and leave the tower at will.
In Suite du Merlin ("Merlin's Continuation", 1240) and Malory's Book IV of Le Morte d'Arthur (1469), Merlin met and had fallen in love with the Lady of the Lake named Niniane (or Viviane, while Malory called her Nimue), after Arthur and Guinevere's wedding. Niniane did not like Merlin at all, because she thought that the wizard was the son of a devil.
Niniane should not be confused with the Lady of the Lake, who gave Excalibur to Arthur, because Balin had murdered her in the king's early reign. See New Sword and Balin about the other Lady of the Lake.
She used Merlin's love, so that he would teach her his magic. In return, for the lessons in magic, Niniane offered to return his love, was nothing more than a subterfuge to gain power to trap the wizard. Merlin had also built her home at Lake of Diana, within the forest of Broceliande, probably in Brittany. With his power, he hid her domain from mortal eyes, so that anyone who travelled by, would only see the lake instead of her home.
At the dolorous forest of Broceliande (some called it Darnantes), Niniane used Merlin's own magic against the sorcerer; she entombed Merlin in a rock.