This is an excellent book, probably the best of the Prydain Chronicles.

A synopsis:
Taran (or Taran Wanderer as he is called by the people whom he met in the novel of the same name), returns to Caer Dallben to find that the sword of power Dyrnwyn has been robbed from the Prince of Don, Gwydion. It is determined that Arawn the Death-Lord himself stole the sword and took it back to Annuvin (his headquarters).

Taran, along with his pseudo girlfriend Eilonwy, his side kick Gurgi, the former giant Glew, King Rhun of Mona and Fflewddur Fflam set out to Caer Cadarn, where they encounter the evil Magg and get caught. King Rhun is killed in the battle to save Taran and Company.

They then set out for Caer Dathyl and are met by the High King himself, Math (son of Mathonwy). The ruler of the western domains, Pryderi, arrives and announces he is in an alliance with Arawn and requests the surrender. In the ensuing battle, Caer Dathyl is destroyed and Math is killed.

A plan is hatched to stop the Cauldron Born (mute, zombie warriors) from returning to Annuvin and regenerating their power. They travel through the mountains, battling at various times with the Cauldron Born and losing many people, such as Taran's friend from Caer Dallben, Coll and Llonio, his friend from the East.

As the story draws to its climax, they reach Annuvin and, hidden under a rock lies the sword of Dyrnwyn. Taran unsheathes the blade and destroys the Cauldron Born.

In the ensuing battle, Magg is melted by wearing the Iron Crown of Arawn, Achren (evil enchantress) dies, Arawn is destroyed by Taran.

After this, Taran is offered a space aboard the ship of the Sons of Don. He accepts, but changes his mind. This, as prophecised by the Book of Three, means that he becomes the High King of Prydain.

He and Eilonwy get married and the Sons of Don take off to the summer country.

And that is the book. It is an amazing read and I suggest you pick it up soon.