Lady of the Lake is a Goddess or magical and immortal being who resided in Britannia.
She is the original guardian of the powerful sword Excalibur. When he was returned in time, King Arthur looked for her, she recognized him as the prophesied bearer of the sword and gave him the sword, although technically there were a thousand winters too early. (HTLJ: "Once Upon a Future King")
Lady of Lake
Lady of Lake and Excalibur
Excalibur and his Guardian
Lady of Lake