Patricia McKillip, The Riddle-Master of Hed (1976).
Long ago, the wizards had vanished from the world, and all knowledge was left hidden in riddles. Morgon, prince of the simple farmers of Hed, proved himself a master of such riddles when he staked his life to win a crown from the dead Lord of Aum. But now ancient, evil forces were threatening him. Shape changers began replacing friends until no man could be trusted. So Morgon was forced to flee to hostile kingdoms, seeking the High One who ruled from mysterious Erlenstar Mountain. Beside him went Deth, the High One’s Harper. Ahead lay strange encounters and terrifying adventures. And with him always was the greatest of unsolved riddles; the nature of the three stars on his forehead that seemed to drive him toward his ultimate destiny.”