The Grey King by Susan Cooper

Description

There is a Welsh legend about a harp of gold, hidden away within a certain hill, that will be found by a boy and a white dog with silver eyes—a dog that can see the wind.

Will Stanton knows nothing of this when he comes to Wales to recover from a severe illness. But when he meets the strange boy Bran and his white dog, memory wakes in Will. For Will is the last-born of the Old Ones, immortals dedicated to saving the world from the forces of evil, the Dark. It is his task to use the golden harp to wake the six who must be roused from their long slumber in the Welsh hills to prepare for the last battle between the Dark and the Light. But first, with help from Bran, Will must face his most terrifying opponent yet: the Grey King.