The High King (The Chronicles of Prydain, #5)

When the sword Dyrnwyn, the most powerful weapon in the kingdom of Prydain, falls into the hands of Arawn-Death-Lord, Taran, Assistant Pig-Keeper, and Prince Gwydion raise an army to march against Arawn's terrible cohorts. After a winter expedition filled with danger, Taran's army arrives at Mount Dragon, Arawn's stronghold. There, in a thrilling confrontation with Arawn and the evil enchantress Achren, Taran is forced to make the most crucial decision of his life.