A key character in C. S. Lewis's Narnia series, Queen Jadis appears in multiple books under different guises.
In her first incarnation, she appears in the first book of the series The Magician's Nephew as Queen Jadis of Charn, a dead world that the main characters have accidently traveled to with the use of magic rings. She is described as a very tall woman with long red hair, and very strong. She was at war with her sister for the throne of Charn, and when all her armys had been defeted, she used the deplorable word. ""That was the secret of secrets," said the Queen Jadis. "It had long been known to the great kings of our race that there was a word which, if spoken with the proper ceremonies, would destroy all living things except the one who spoke it. But the ancient kings were weak and softhearted and bound themselves and all who should come after them with great oaths never even to seek after the knowledge of that word. But I learned it in a secret place and paid a terrible price to learn it. I did not use it until she forced me to it. I fought to overcome her by every other means. I poured out the blood of my armies like water -" (The Magicians Nephew) When the children (who are the main characters in the book) ask her why she killed all those innocent people she replies quite matter of factly "I was the Queen. They were all my people. What else were they there for but to do my will?"
After forcing the children to take her back to Earth where she makes some trouble, they are transported to yet another world, Narnia. They witness the birth of a world, created by Aslan the great lion (and god figure of the books). Jadis makes this her opportunity to escape into a new world."Before the new, clean world I gave you is seven hours old, a force of evil has already entered it; waked and brought hither by this son of Adam," says Aslan (p.126). At the end of the book she eats an apple from a special tree that makes her immortal.
Queen Jadis becomes the nemesis of Narnia, showing up again in The Lion, the Witch and the Wardrobe as the White Witch. Craving the power of a throne and possessing the magic, the irresponsibility and the sefishness to use it, she covers Narnia in snow and drains all happiness from the land, "always winter and never Christmas".
In an attempt to thwart a prophecy telling of four children (two sons of Adam, two daughters of Eve)who will bring Aslan back to the land and end her tyrannical reign, she tries to capture the four main characters of the book, Edmund, Lucy, Peter and Susan.
By the end of the book her plans have been thwarted by the children with the help of Aslan, and she is killed in the final battle.
Or did she?
Further on in the series in Prince Caspian the old hag tells the Prince that Jadis can never really die but can be summoned up if they want her. It is believed that the old hag actually is Jadis, and her dialoge with the Prince is her attempt to get him to restore her powers.
In the Silver Chair Jadis comes back as the Lady of the Green Kirtle. She is a powerful sorceress who can shapeshift into a snake and kidnapps the Prince (not Caspian) She too is thwarted by children (once again, a different set of children). There is some argument over whether Jadis and the Lady are the same person. Some believe Jadis' death in The Lion, The Witch and the Wardrobe was final, other believe that her various incarnations represent Jadis and her decendents but are not actually the same people. I am going with the character bios at the begining of each book which mention Jadis coming back in the Silver Chair (although at first glance there is no sign of her).
Jadis is used as an allegory for evil throughout the Narnia books. Lewis' never explicitly makes her the devil (he is too aware of gender roles for that), but continually brings her back to tempt the children and the denizens of Narnia, and to fight with Aslan. Her lineage is questionable, but it is commonly thought that she is the daughter of Lilith, Adams first wife and a giant (her race is the Jinn)
In the feature film from Disney, The Chronicles of Narnia: The Lion, the Witch and the Wadrobe, the White Witch is played by Tilda Swinton.