The Eyes Of The Dragon

The passage through the castle is dim, sensed by few and walked
by only one, Flagg knows the way well. In four hundred years, he has
walked it many times, in many guises, but now the passage serves
its true purpose. Through the spyhole it conceals, the court magician
observes King Roland--old, weak, yet still a king. Roland's time is
nearly over, though,and young Prince Peter, tall and handsome, the
measure of a king in all ways, stands to inherit the realm.

Yet a tiny mouse is enough to bring him down, a mouse that chances
upon a grain of Dragon Sand behind Peter's shelves and dies crying
tears of fire and belching gray smoke. A mouse that dies as King
Roland does. Flagg saw it all and smiled, for now Prince Thomas, a
young boy easily swayed to Flagg's own purposes, would rule the
kingdom. But Thomas has a secret that has turned his days into
nightmares and his nights into prayed-for oblivion. The last
bastion of hope lies at the top of the Needle, the royal prison
where Peter plans a daring escape. . .

The Eyes Of The Dragon
Stephen King
1987