The Farthest Shore

Ursula K. Le Guin

A young prince, heir to a long-empty throne, comes to the island of mages with word that magic is failing. He and the archmage set out across the sea to investigate, finding at the edges of the world the fatal consequences of mages' misled desires to cheat death. They journey into and through the land of the dead to defeat the man responsible, restoring hope and wisdom to the world.