Power Over Life and Death

"To save a life, there must be a death. The balance of the world must be restored."

- Nimueh to Merlin

The power over life and death is a form of magic that lies at the very heart of the Old Religion. It can be used to revive people who are dying or to create an entirely new life, as long as another life is given in return.

Rules and Effects
The power of life and death could be used both to save a life and to create one, as long as another life was given in return. It could even be used to allow a barren woman to conceive (Excalibur).

However, the use of this magic carried a heavy price. According to Nimueh, not even a High Priestess had the power to mirror life itself while giving nothing in return. As such, another life had to be sacrificed in order to restore the balance of the world, and once a life-for-a-life bargain was struck it could not be undone (Le Morte d'Arthur).

This made the power over life and death a very dangerous form of magic to use, mainly because of its fatal and potentially unpredictable results. For example, when Uther Pendragon asked Nimueh to use this power to allow his wife Ygraine to conceive, Ygraine's life was taken in return. Grief-stricken, Uther blamed magic itself for her death and instigated the Great Purge (Excalibur).

According to Nimueh, Ygraine's death was not her choice but merely the law of magic. However, it has been implied that she may have been more culpable than she cared to admit. When Arthur was bitten by the Questing Beast, Merlin went to Nimueh and bartered his own life for the prince's but his mother was nearly killed instead. When he confronted her about this, she replied, "The Old Religion does not care who lives and who dies. Only that the balance of the world is restored." This may imply that choosing who was sacrificed for whom was entirely up to the user's discretion, which is supported by the fact that she had no trouble ensuring that Gaius' life would be traded for Hunith's (Le Morte d'Arthur).

Methods of Use
The power over life and death is a very powerful form of magic that could only be achieved by powerful sorcerers. Merlin and the High Priestess Nimueh are the only known individuals to have mastered it. The Questing Beast also carried this power.

As servants of the Old Religion, it has been speculated that all High Priestesses possess this power, but neither Morgause nor Morgana ever demonstrated any knowledge of it.

There are many ways in which the Old Religion (or those who serve it) have control over life and death. Nimueh was shown to use the Cup of Life to both revive the dying and to kill an intended sacrifice. She also presumably called upon the Old Religion to strike another sacrifice with an incurable disease.

Merlin, however, used neither of these methods. When he discovered that Nimueh had sacrificed Gaius, he summoned a storm and struck down the High Priestess with lightning, trading her life for his. It is unknown whether he instinctively used his magic to do this or called upon the power of the Old Religion (Le Morte d'Arthur).

Exceptions to the Rule
Nimueh once told Merlin that to save a life, another life had to be given in return in order maintain the balance of the world. However, there are apparently exceptions to this rule, as magic has been used to heal a number of potentially fatal injuries without any known consequence: Considering these exceptions, it's possible that power over life and death (and therefore a sacrifice) is only needed to revive a victim when the power itself has caused their death. For example, because the Questing Beast was a creature conjured by the powers of the Old Religion, only the same ancient magic had the power to cure its bite (Le Morte d'Arthur).
 * It is unknown whether the power of life and death was involved in the creation of the Afanc, but no one is known to have died from it (The Mark of Nimueh).
 * Edwin Muirden used magic to extract his beetle from Morgana's ear (thus curing her and saving her life) and Merlin later did the same for Uther. Nobody is known to have died as a result (A Remedy to Cure All Ills).
 * Balinor used magic to heal Arthur's badly infected battle wound. Nobody is known to have died as a result (The Last Dragonlord).
 * Kilgharrah used an enchantment to heal Merlin after he was stung by a Serket. Nobody is known to have died as a result (The Tears of Uther Pendragon).
 * Taliesin used magic to heal Arthur after he was struck by a poisoned arrow. Nobody is known to have died as a result (The Crystal Cave).
 * Merlin used magic to heal Morgana's skull fracture. Nobody is known to have died as a result (The Crystal Cave).
 * The Druids used the Cup of Life to bring Sir Leon back from the brink of death. Nobody is known to have died as a result (The Coming of Arthur).
 * The Vilia used their magic to heal Merlin of the effects of the Dorocha's touch. Nobody is known to have died as a result (The Darkest Hour).
 * As no one else has ever survived a Dorocha attack, it's possible that Merlin's magic was what saved him in the first place.
 * Merlin attempted to heal Uther of his fatal stab wound. There was no implication that a sacrifice was needed for this (The Wicked Day).
 * Merlin used his magic to create a butterfly. It is unknown whether the power over life and death was involved in this, but no one is known to have died from it (The Diamond of the Day).

There is no indication that the power of life and death is involved in Necromancy, nor that is has any relation to magic that is used to reanimate corpses or skeletons, unnaturally prolong life, preserve the soul, or gain immortality.