From C.S. Lewis' The Lion, the Witch, and the Wardrobe:

(After his Talking Beasts rescue the traitor Edmund, the evil White Witch parlays with the Narnian Lord Aslan)

"Have you forgotten the Deep Magic?" asked the Witch.

"Let us say I have forgotten it," answered Aslan gravely. "Tell us of this Deep Magic."

"Tell you?" said the Witch, her voice growing suddenly shriller. "Tell you what is written on that very Table of Stone which stands beside us? Tell you what is written in letters deep as a spear is long on the trunk of the World Ash Tree? Tell you what is engraved on the sceptre of the Emperor-Beyond-the-Sea? You at least know the magic which the Emperor put into Narnia at the very beginning. You know that every traitor belongs to me as my lawful prey and that for every treachery I have a right to a kill."

If the Deep Magic were opposed, Narnia would perish in fire. To appease it, Aslan offered himself as a sacrifice on Edmund's behalf and was killed on the Stone Table. Deeper magic from before the dawn of time, however, mandated that any willing victim killed in a traitor's stead would be resurrected, as was Aslan.