Board Thread:Questions and Answers/@comment-28430223-20160511160512/@comment-37017073-20181023222436

I think that in most cases the life for a life policy was only a requirement when it came to giving life, and not when saving a life. In the case of Arthur and Ygraine, for example, Nimueh was basically using magic to create an entirely new life, a life that could not naturally come into existence without her direct intervention. As such, the idea that she would have to pay for that new life by sacrificing an already existing one - effectively trading one's place in the world for the other's and maintaining a balance - makes sense. Nimueh herself says as much in Excalibur: "That is the law of magic. To create a life, there had to be a death. The balance of the world had to be repaid."

When it comes to saving a life, though, there's little evidence that any such rule exists. Many people's lives are saved using magic over the course of the series (both before and after the events of Le Morte d'Arthur), and the only time the life for a life policy was anywhere to be seen was in regards to the Cup of Life. And even then it only seemed to apply when Nimueh was in possession of it, which is why I think that the requirement of giving a life to save a life was Nimueh's policy rather than the Cup's. This is somewhat supported by The Darkest Hour, specifically by the conversation between Merlin and Kilgharrah about a sacrifice being required to heal the veil: "I know that the spirit world demands a sacrifice." "It demands nothing. It is the Cailleach, the gatekeeper of the spirit world, who asks such a price." To me, at least, this exchange suggests that a sacrifice was not an essential requirement to fixing the veil, but rather the price that the Cailleach demanded before she would do so. It was likely the same with Nimueh and the Cup of Life. She was the one in possession of it, therefore she could charge whatever price she wanted for its use. And what she wanted was a life for a life.