The Lion, the Witch, and the Wardrobe
How we cite our quotes:
"But where is the fourth?" asked Aslan.
"He has tried to betray them and joined the White Witch, O Aslan," said Mr. Beaver. (12.17-18)
It's interesting that this conversation takes place. Aslan pretty much knows everything that is going on in Narnia, and he definitely knows what's up with Edmund and the Witch, so we assume that asking about the fourth child is just a formality. It needs to be said outright that Edmund is a traitor.
"You have a traitor there, Aslan," said the Witch. Of course everyone present knew that she meant Edmund. But Edmund had got past thinking about himself after all he'd been through and after the talk he'd had that morning. He just went on looking at Aslan. It didn't seem to matter what the Witch said. (13.37)
Edmund's conversation with Aslan dispels all the after-effects of his betrayal. Edmund has begun to change radically and forever, and part of that change is that he's not thinking about himself all the time.
"You know that every traitor belongs to me as my lawful prey and that for every treachery I have a right to a kill." (13.41)
The existence of betrayal in Narnia is what gives the Witch a basis for her power.