Lady of the Lake, in Arthurian legend, a misty, supernatural figure endowed with magic powers, who gave the sword Excalibur to King Arthur. She inhabited a castle in an underwater kingdom. According to one legend she kidnapped the infant Launcelot and brought him to her castle where he lived until manhood. She has been identified variously with Morgan le Fay and Vivien. The poem The Lady of the Lake, by Sir Walter Scott, is based on a totally different legend.