The Hag is an antagonist in the book and movie of Prince Caspian. She is a loyal follower of the White Witch Jadis and the leader of her remaining, scattered forces.
Because the Hag looks like a normal old woman, it was easy for her to disguise herself as a possible ally of the Narnians. But they smelt a rat when she began talking admiringly of Jadis, and Caspian suddenly saw her for what she was when the torches were lit. The Hag fought Caspian, trying to strangle him, and was decapitated by him.
In the movie, her role is much bigger, as she is given the job of resurrecting Jadis, and succeeds in resurrecting her ghost. Jadis is not corpereal enough for full resurrection and needs human blood to finish the task. When Edmund stabs the ice portal and banishes the Witch's ghost, the Hag is also killed in the event.