The Black Cauldron is the second book in The Prydain Chronicles by Lloyd Alexander.
Synopsis
At the council at Caer Dallben, Gwydion explains that Arawn is using the black cauldron to create more of the Cauldron-Born, his servants who cannot die. He then explains his plan to steal the cauldron and destroy it.
However, when they get to where the cauldron should be, it isn't there. A fair folk reveals that it was taking by Orddu, Orwen, and Orgoch, and Taran, Eilonwy, Fflewddur and Gurgi set out to find it.
When they reach the marshes, the witches agree to trade the cauldron for a valuable item that they possess. Each protagonist in turn offers his most precious item, but in the end Taran is forced to give them the Brooch of Adaon. Afterwards, it is revealed that the cauldron can only be destroyed if a living human is put inside. However, then human too will die.