NIMUE
***No image in the Marvel Universe***

Real Name: Vivienne (also Vivian)

Identity/Class: Human (Camelot era) magic user

Occupation: Royalty, Sorceress

Group Membership: None

Affiliations: Morgan Le Fey (former mistress and mentor), Merlin (former mentor and lover);
uncertain connection to Niamh

Enemies: Merlin

Known Relatives: Diones (father), Lunette (cousin), a Sicilian siren (possibly her mother)

Aliases: Niniane, Niviene; possibly the Lady of the Lake (see comments)

Base of Operations: the Forest of Broceliande in Brittany (where, many French tales, she imprisoned Merlin); Camelot Era

First Appearance: (first appearance in literature) ???
(mentioned in the Marvel Universe) Iron Man I#150 (September, 1981)

Powers: Uncertain. Nimue was apparently a powerful sorceress, having trained under the two most powerful magic users of her era. Her exact abilities are unknown, but she could accomplish a spell powerful enough to imprison Merlin for ages.

History: Vivian is credited with being the daughter of Diones, a vavasour (land-owner) who served the Duke of Burgoyne, and a Sicilian siren. She first met the wizard Merlin when she was twelve years old and he promised to teach her some of his magic. By time she was eighteen, after serving as both a handmaiden and a pupil in the sorcerous arts to Morgan Le Fey, she had also become the wizard's lover and pupil. Morgan Le Fay showed the jealous Nimue, obsessed with keeping Merlin faithful, a spell to keep her lover under her power, but the spell actually came close to imprisoning them for millennia and thereby keeping the both of them out of the way of foiling her plans with King Arthur. Ultimately, she did turn upon Merlin, imprisoning the archmage in a tree (shades of the Norse myths of Loki's imprisonment to be seen here), a crystal cave, or a fortress of air according to various traditions.

In some stories, Vivian ended up replacing the Lady of the Lake as the protector of Excalibre after the goddess (Niamh) was killed by Sir Balin, whilst Merlin continued in his mystic slumber awaiting some future day when Arthur and Britain would need his powers once more.

(Iron Man I#150-BTS) - As related by King Arthur:
It was then that Merlin, succumbing to a fate decreed by the goddess, Nimue, lay himself in his own grave, to sleep for centuries, until bidden once more to wake.

Comments: Created by I have no idea.
Adapted (or at least mentioned) in the Marvel Universe by David Michelinie.

The above history is a simplified merger of some of the stories of Nimue and as hence omits some of the details and motivations involved.

In some versions of the Arthurian legends, particularly the Vulgate, there were two separate women/goddesses: Niamh and Vivienne, both of whom also received the name Nimue, who is the Lady of the Lake. In several tales, the identity of Niamh/Nimue merges with that of Nimue/Vivienne, the love of Merlin. In the stories where Niamh was murdered by Sir Balin, Nimue/Vivienne replaced her as the Lady of the Lake.
If only for the purposes of clarifications, the Appendix will list the Lady of the Lake as Niamh/Nimue, and as a separate being from Nimue/Vivienne. The actual situation in the MU is unlikely to ever be revealed.
However, it may well be that Nimue/Vivienne is, in fact, the Lady of the Lake seen in the MU. If so, then her character bibliography can be found at that profile. If they are indeed separate, then Nimue/Vivienne has only been mentioned and not seen in the MU.
My guess would be that no one at Marvel knows there are two characters. I'm fairly certain that anyone using the Lady of the Lake intends it to be the same person who gave Arthur Excalibre in the the first place. But I would like to see more of Nimue/Vivienne.

Further explanation on the division b/t Niamh and Nimue, courtesy of Greg O:
    Nimue is often referred as the young enchantress that seduced Merlin. If she was once Morganna Le Fey's pupil, then later Merlin's pupil and eventual captor how could she be the same person that Merlin brought Arthur to in order to obtain Excalibur? It just doesn't add up.
    In addition, it just doesn't make sense for Morgan's pupil to have sabotaged her work and saved Arthur's life. I think that had to be Niamh.

To be honest, with many of the myths, the more you try to pin down who's who and who did what, the more confused you get. I tried to research this more myself and kept getting further from what I knew/thought was in line with the MU character, so I stopped...and now I'm more confused than ever--Snood.

Arthur did indeed return in the time of need of England, in the pages of the Marvel UK Hulk Comic, to oppose the forces of Necromon.

Dr. Vivian Morgan was an oceanographer who got possessed by Morgan le Fay @ Namor#58-62. Her name almost certainly draws on the names of both Morgan le Fay (or the goddess Morgan, from which Nimue is derived) and Nimue/Vivienne.

Many, many thanks to Will U and Greg O, two of the Appendix's myth-masters, for supplying the historical/legend version of both the Lady of the Lake and Nimue/Vivienne. I also gained additional information from: Lady of the Lake, Celtic Gods and Goddesses, Encyclopedia of the Celts, and a few others, which is listed below, although it doesn't seem to have a whole lot to do with the Lady of the Lake

Mythologically, Nimue (pronounced Nim-oo-ay) was a lessor Celtic Moon Goddess; cognate with the Greek Nemesis and Diana of the Grove. Her name meant "fate" and "she who lives" and was said to reside in the Fairy wood of Broceliande. She was also connected to the Goddess Morgan.

Archetypically, Nimue and the Lady of the Lake represented the primal initiation into the Otherworld. She reigned over knowledge, was the foster mother, and the mistress of wisdom.

CLARIFICATIONS:

• Lady of the Lake-uncertain connection