Tennyson's Idylls of the King are widely regarded as some of the best modern stuff (if not the best stuff) on modern Arthurian legend. The whole set of poems was written more or less as some sort of political point about how much Tennyson as a good post romantic poet hated the Industrial age.  However, the poems are delightful.  Vivian (Niniane's evil alter ego) gets to be wonderfully villainous, exactly all that a woman should not be.  (Which is kind of a pity as she is intelligent, ambitious, beautiful, etc.  Of course, she's not real nice and a slut, but whatever.)  She seduces Merlin in the poem Merlin and Vivian or just Vivian and gets a huge number of great lines about her and a marvelous story that really must be read by anyone who likes her at all.  I think that this may even be the story that originated Merlin being trapped inside a tree, although it's hard to tell. It is certainly one of the most popular, best known, and best written recordings of the whole Merlin and Vivian story.  She also appears, briefly, in Guenivere where she plots against Guenivere with Mordred.  What a marvelous joining of villains!  I'd recommend reading the whole thing, although it's hard to take too much of the rather dense poetry at once.

Dates:
Idylls of the King: 1859