The Silver Chair

"We've got to go north," said Edmund, "and reach the ruins of a giant city." Prince Rilian, Caspian's beloved son, has mysteriously disappeared, and Jill and Eustace are brought into Narnia by Aslan, the great Lion, to find him. But an evil enchantress has dire plans for the prince, and for Narnia This is the sixth adventure in the exciting Chronicles of Narnia.