Nimueh uses the Cup of Life.
The Cup of Life can be used by a High Priestess or another extremely powerful warlock or witch for different purposes.
Give a life
The Cup of Life, blessed by many centuries of powerful sorcery, contains the very secret of life itself. Because of this, it can be used to slightly alter the balance of the world by creating a new life. A life must be given in return to re-stabilize this balance.
Uther, when he realised that his wife Igraine was barren, asked the High Priestess Nimueh to use the power over life and death and help the Queen to conceive. Igraine's life was taken when Arthur was born.
The power over life and death can also be used to save someone who's on the very point of death and there's
Nimueh pours water from the Cup into a vial.
no cure for his illness. Even in this case, the Old Religion demands a life. Merlin, when Arthur was bitten by the Questing Beast bargained his life to save the Prince. He went to the Isle of the Blessed where Nimueh collected rain water into the Cup and then poured it into a vial. When Arthur drank from it, he recovered. To save the life of his apprentice, Gaius himself went to the Isle and convinced Nimueh to take his. The High Priestess chanted this spell on Gaius, who was lying next to an altar, while holding the Cup: "Ic, séo héahsácerd, þe ácwele strengþe ealde æwfæstnesse" (Le Morte d'Arthur).
Save the dying
The Cup's healing power can easily save someone who's been deadly wounded. If he drinks water from the Cup,
The Cup in the hands of the Druids.
he'll live. In this case, the Old Religion doesn't require a life in return. When Sir Leon was attacked by Cenred's men, he was saved by the Druids. In their cave, they all chanted a spell ("Buthed gwared. Hrag pob ailed. Hrag pob evnis. Boyd un thilis") and made him drink water from the magical chalice. The wounds on his body immediately disappeared (The Coming of Arthur: Part One).
Immortality
Morgauses binds Cenred's army to the Cup of Life.
If someone's blood is collected in the Cup and the chalice is enchanted ("Gegadre anne here fram þisse bune ond heora blod. Swa þæt hie ne abygð ond ne swilteaþ naht, ac leofaaþ a on ecnesse") by a High Priestess, this person will become immortal as long as his blood remains in the Cup. There's a price to pay: he will become the living dead, entirely under the sorceress' control, and will vanish when the Cup is emptied of the blood it contains. Morgause, when Cenred's men found the Cup, did this to gain control over Cenred's army, making it immortal. With these men, she and Morgana conquered Camelot, but were later defeated. (The Coming of Arthur).