A Priestesses' Burden
If there's one thing that can destroy a life was desiring more than what you could have. Nimueh knew that well enough, she had felt it on her skin the terrible consequences of granting someone's desire when they couldn't deal with the consequences.
Still, she would not deny the boy what he wanted. Deep down, she could understand that Emrys – Merlin, as he was called – felt honor bound to save Arthur's life. It was something that had torn her too, allowing it to be taken after how much she had struggled to create it, but Uther had tainted the boy with his ridiculous views and he now would not be the man he was supposed to become.
She summoned the rain and raised the cup, gathering the water of life inside it. She felt the power pass through her body, and for that moment, it felt as if all the wounds of the world could be healed by it.
Merlin had pledged his life for Arthur's, but she knew it would never be his life that would be claimed. Merlin's life wasn't his own to offer, and she knew the law would claim him something just as precious as he saved the man he was bound to serve. Nimueh tried to warn him, but he didn't take notice of her words, confident that it would be him that would die.
Men hardly ever listen – they think too much of their own and forget that they cannot exist without the women that surrounds them. It was them that had the power of life in their hands, and them that would suffer the consequences.
Nimueh was a witch, a priestess and an enchantress, but she was only a tool in the hand of the Gods and had learned to accept that fate wasn't in her hands. She knew it too well, but it was something that Merlin would have to learn the hard way, and there was nothing she could do for him as she watched him go, but to hope that he'd accept it better than Uther had.
(He didn't, and it coasted her life. Her last thought before death was that men don't ever change, don't ever learn to loose, and the women always paid the price for it.)