Tehanu (Le Guin, Ursula K., Earthsea Trilogy.)

Years before, they had escaped together from the sinister Tombs of Atuan - she, an isolated young priestess, he, a powerful wizard. Now she is a farmer's widow, having chosen for herself the simple pleasures of an ordinary life. And he is a broken old man, mourning the powers lost to him not by choice. A lifetime ago, they helped each other at a time of darkness and danger. Now they must join forces again, to help another - the physically and emotionally scarred child whose own destiny remains to be revealed.