The High King is the fifth book in The Chronicles of Prydain series by Lloyd Alexander. It is the most overtly epic of the books, the most purely high fantasy, and the apex of the overall plot of the Chronicles, in which nearly every character previously established in the four preceding books has some part to play.
When the legendary sword Dyrnwyn was stolen by Arawn Death-Lord, Taran of Caer Dallben and his Companions undertook an arduous mission to regain the blade. In the quest, Taran rallied the men of the Free Commots to the aid of Gwydion and the Sons of Don against all the evil forces of Annuvin. The final confrontation would shatter all that Taran knew, and change the land of Prydain forever.