Arthurian legends have a speck of truth in them when applied to the magical world, there was a Merlin and a Morgana and a King Arthur. The similarities stop there as the history was lost to time. One fact about this is that Nimue isn't a person but a title. The title Nimue was a title given to the first ever Witch to form a contract with the Fae, it was supposed to be a generational title but the last person to hold the title of Nimue never had any children, and coincidentally she was the daughter of Merlin. That's why you don't see summoning magic or heirs of Merlin walking around. She decided that the power to summon Fae was too dangerous to be in the hands of the common witch or wizard and let the Branch die with her, being the first witch since the first Nimue to form a contract with the Fae (as it happens any spirit elemental or not counts and part of the Fae court) I inherited the title. The original Nimue first contracted with a water spirit that she summoned next to the black lake in Hogwarts, so the title she gained was the Lady of the Lake. Since immediately after contracting with the fae I empowered them with the power of hell the title I therefore Gained was Nimue Aeter Ignis or Nimue of Eternal Fire. The title doesn't exactly translate because it isn't meant to be translated, it has a meaning that becomes the name so when I commune with spirits I'm known as Nimue Aeter Ignis as if that was my first middle and last name.