The Witch King by Maeve Henry, 1987.

Robert is the son of a fisherman in a small village by the sea. All his life, he’s grown up hearing stories of a mysterious City to the south that was fashioned ages ago from a silver tree. Wondrous things are said to happen there. People from the City used to visit the Witch Women in Robert’s village for advice, but no one has come there from the City in a long time, and people now doubt whether the City really exists. People have even stopped believing in the Witch Women. However, Robert believes in the old stories, and he wants desperately to see the City for himself.

The stories say that someday the Witch King will come to the City and plant a seed so that a new tree will grow. Robert wants to see this story come true, and Granny Fishbone, a strange old woman who tells Robert the old stories, says that the time of the Witch King is close. She gives him a pendant in the shape of a fish and tells him that it came from the City and that it is time for Robert to take it back there.

With some misgivings, Robert’s family lets him set off on his seemingly crazy quest of finding the City. To Robert’s surprise, he is met on the road by the Royal Wizard of the City. As Granny Fishbone said, the time of the Witch King is near, and there is trouble in the City. The Spell that controls the City is failing, and the King has been seeking a remedy for the problem.

The City has become corrupt because a usurper murdered the rightful king years ago and took over the City. He had his wizards cast a spell on the City that let him control everything in it, and he manipulated historical accounts to make it seem as though he was the founder of the City himself. Robert knows differently because Granny Fishbone is really one of the Witch Women, and she told him the true story about the miraculous tree and the founding of the City. The king’s grandson, David, is also corrupt and wants to learn to control the spell over the City for his own power. He believes that he is the prophesied Witch King, and he tries to convince Robert of it so that he will help him.

When Robert learns that David’s plans for controlling the spell may mean sacrificing his own sister, Princess Sophie, who Robert loves, Robert does his best to thwart his plans by telling the king and the Royal Wizard.  Although David at first seems to have the upper hand, he is not the true Witch King. Granny Fishbone was correct that the time of the true Witch King has come, and Robert has a much bigger role to play in the story than he thinks.  As the ancient prophesy says, the City will have to be destroyed in order to be saved.