The Deep Magic

The Deep Magic refers to a set of laws placed into Narnia at the time of its creation by Emperor-beyond-the-Sea. These laws were inscribed on the Stone Table, the firestones on the Secret Hill, and the sceptre of the Emperor-beyond-the-Sea.

The law stated that the White Witch, Jadis, was entitled to kill every traitor, and if she was denied this right, all of Narnia would be "overturned and perish in fire and water".

However, unknown to Jadis, a deeper magic from before the dawn of Time existed, which said that if a willing victim who had committed no treachery was killed in a traitor's stead, the Stone Table would crack, and Death would start working backwards. When Jadis made a claim on Edmund Pevensie's life, Aslan used this Deeper Magic to save him.

Trivia
Given the very limited descriptions of the Deep Magic, it's unclear exactly how the system works, or how pervasive it actually is in the World of Narnia outside of this one specific use. As with much else in The Chronicles of Narnia, Christian allegory is an overriding theme, and scholarly interpretation of the Deep Magic has also been in that context.

In his book The Magical Worlds of Narnia: A Treasury of Myths, Legends, and Fascinating Facts, David Colbert outlines a number of religious elements that may have influenced C.S. Lewis' writing on this particular topic:


 * The Deep Magic can be seen as an allegory for "Natural Law", a philosophical concept that claims there are innate rules of right and wrong understood by all human beings. The source of this law is said to be above and outside of humans (i.e. God) and is a concept central to all civilizations.
 * Jadis' claim on the blood of traitors relates to her role as an allegorical form of Satan. As told in the Bible's Book of Job, Satan was given the right by God to test the worthiness of humans, and if they failed that test - as Edmund apparently did - Satan could claim them as his own.
 * Aslan himself is very clearly an allegory for Jesus Christ - he is even called the Son of the Emperor-beyond-the-sea - and his knowledge of the Deeper Magic is something that Jadis (a.k.a. Satan) does not possess. By allowing himself to be sacrificed in Edmund's place, Aslan is mirroring the Crucifixion, where Christ offered up his life for the sins of humankind (Aslan's resurrection afterwards only reinforces this point).