In Cursed: Nimue is the protagonist. She’s a fey of the Sky Folk who, as the daughter of the priestess Lenore and Merlin, has stronger powers than her fellow fey that have seen her branded a witch. Over the course of Season 1, she becomes queen of the fey, leading her people with the sword of power.
In legend: Nimue is the Lady of the Lake. She’s very much a background figure, and there are different versions of the Lady of the Lake that play different roles depending on the version of the story. The Lady of the Lake is usually associated with the sword, Excalibur, but only in giving it to Arthur, not wielding it herself.
In one variation, the Lady of the Lake is Lancelot’s foster mother. In some versions of the story, Merlin is in love with her, and she imprisons him and takes his place as Arthur’s advisor. But perhaps the most enduring image of Nimue is her taking back Excalibur and escorting Arthur to the magical isle of Avalon after he is mortally wounded in battle.