and called him devil's son.

Then, on a certain time, Merlin said to Arthur: "The time draws near
when ye shall miss me, for I shall go down alive into the earth; and
it shall be that gladly would ye give your lands to have me again."
Then Arthur was grieved, and said: "Since ye know your danger, use
your craft to avoid it." But Merlin answered: "That may not be."

Now there had come to Arthur's court, a damsel of the Lady of the
Lake--her whose skill in magic, some say, was greater than Merlin's
own; and the damsel's name was Vivien. She set herself to learn the
secrets of Merlin's art, and was ever with him, tending upon the old
man, and with gentleness and tender service, winning her way to his
heart; but all was a pretence, for she was weary of him and sought
only his ruin, thinking it should be fame for her, by any means
whatsoever, to enslave the greatest wizard of his age. And so she
persuaded him to pass with her over seas into King Ban's land of
Benwick, and there, one day, he showed her a wondrous rock formed by
magic art. Then she begged him to enter into it, the better to declare
to her its wonders; but when once he was within, by a charm that she
had learned from Merlin's self, she caused the rock to shut down that
never again might he come forth. Thus was Merlin's prophecy fulfilled,
that he should go down into the earth alive. Much they marvelled in
Arthur's court what had become of the great magician, till on a time,
there rode past the stone a certain Knight of the Round Table and
heard Merlin lamenting his sad fate. The knight would have striven to
raise the mighty stone, but Merlin bade him not waste his labor, since
none might release him save her who had imprisoned him there. Thus
Merlin passed from the world through the treachery of a damsel, and
thus Arthur was without aid in the days when his doom came upon him.