A wily wanton in Arthur's court “who hated all the knights.” She tried to seduce “the blameless king,” and succeeded in seducing Merlin, who, “overtalked and overworn, told her his secret charm” -
The which if any wrought on anyone With woven paces and with waving arms, The man so wrought on ever seemed to lie Closed in the four walls of a hollow tower, From which was no escape for evermore.
Having obtained this secret, the wanton “put forth the charm,” and in the hollow oak lay Merlin as one dead, “lost to life, and use, and name, and fame.” (Tennyson: Idyls of the King; Vivien.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Vivien from Infoplease: