In Medieval Literature
The Lancelot-Grail Cycle provides a back-story for the Lady of the Lake, Viviane, in the prose Merlin section, which takes place before the Lancelot Proper, though it was written later. There, Viviane learns her magic from Merlin, who becomes enamored of her. She refuses to give him her love until he has taught her all his secrets, after which she uses her power to trap him either in the trunk of a tree or beneath a stone, depending on the story and author. Though Merlin, through his power of foresight knows beforehand that this will happen, he is unable to counteract Viviane because of the "truth" this ability of foresight holds. He decides to do nothing for his situation other than to continue to teach her his secrets until she takes the opportunity to entrap and entomb him in a tree, a stone or a cave.
The Post-Vulgate Cycle's second Lady of the Lake is called Ninianne, and her story is nearly identical to the one in the Lancelot-Grail, though it adds her bestowal of the magic sword Excalibur to Arthur. Sir Thomas Malory also uses both Ladies of the Lake in his Le Morte d'Arthur; he leaves the first one unnamed and calls the second one Nimue. Malory's original Lady is presented as an early benefactor of King Arthur who grants him Excalibur when his original sword is damaged. She is later beheaded by Sir Balin as a result of a kin feud between them (she blames him for the death of her brother and he blames her for the death of his mother) and a dispute over an enchanted sword.
Both characters appear in many other episodes of Malory's work. Each time the Lady reappears, it is at a pivotal moment of the episode, establishing the importance of her character within Arthurian literature, especially Le Morte d'Arthur. In that work, she transcends any notoriety attached to her character by aiding Arthur and other knights to succeed in their endeavors. After enchanting Merlin, Malory's Nimue replaces him as Arthur's adviser. She becomes the lover and eventual wife of Sir Pelleas and mother to his son Guivret. After the Battle of Camlann, she reclaims Excalibur when it is thrown into the lake by Sir Bedivere. Nimue is one of the four magical queens who bear the wounded Arthur away to Avalon, a setting tied to the Lady of the Lake in some literary traditions.
A magical being, the Lady of the Lake has encountered some superheroes from around the world, most notably Captain Britain, the Black Knight and their friends. More recently she fought alongside Captain Britain and his team during the Skrull invasion on Brittain and the world. It seemed that she was killed by a Skrull with an anti-magic gun. She however showed up more recently in the pages of Journey into Mystery, so her apparent death was a mistake.
King Arthur throws the sword Excalibur to her, and her sister Nimue jumps in after it, but Vivienne will not let her sister have it and blocks her way.
Vivienne is the spirit caretaker of a mystic realm called the Secret Sea. She found Aquaman near death.
After he was exiled from the oceans and left for dead on Traitor's Reef, she restored his health and unveiled the Secret Sea (an uncharted dimensional reservoir whose ebb and flow is felt in the "World of Man") to him.
Orin was selected to be a conduit for channeling the healing energy of the Secret Sea into our Earthly dimension. His artificial harpoon hand is replaced with magically powered living water, an extension of the Secret Sea and herself.
She is not limited to her realm and can visit her champion in times of crisis.
The Lady of the Lake (also known as the Green Lady) appears in the story arc The Destiny Game.
She is a messenger of the Fates and has the ability to assign and reassign different destinies to particular individuals. Initially a sly and conniving woman, she turns more diligently after two encounters with Bigby Wolf.
It is implied that she later marries Snow and Bigby's cub Ambrose.
She is referred to in The Good Prince; after Flycatcher's heroic deeds in the Homelands kingdom of Haven, he asks Lancelot to throw Excalibur into the biggest lake he can find, and tells him to not be surprised if a "green bejeweled hand" reaches up to grasp Excalibur before it sinks out of sight.
She is also referred to in the Jack of Fables story arc The Bad Prince, when a mysterious old man comes out of the river, carrying Excalibur and says that the "green woman's gift must not fall into evil hands".