The Black Crochan is a large, magical iron cauldron that features prominently in the second book of the Chronicles of Prydain, The Black Cauldron. It is used in the production of Cauldron-Born, the undead soldiers of the Death-Lord Arawn. It may have had other uses, but by the time of the stories, all that it is good for is producing Cauldron-Born.
Originally the property of three witches, Orddu, Orwen and Orgoch, the Crochan was loaned to Arawn. This loan was intended to be for a limited time, but Arawn tried to renege on the deal and keep the Crochan, forcing the witches to act to take it back. It is found by Taran, Fflewddur Fflam, Eilonwy, and Gurgi, who have been sent, along with others, from Caer Dallben to retrieve the Crochan and, hopefully, to destroy it.
Taran eventually convinces the witches to part with the Crochan by trading a magical brooch, formerly the property of Adaon, the son of Taliesin, who had been killed while accompanying Taran and his friends. When Taran tries to destroy it, the witches explain that the only way to destroy the Crochan is for a living man to enter it, knowing that to do so is fatal.
While trying to transport the Crochan to Caer Dallben, Taran loses it to Ellidyr, who craves the glory of bringing it in. Ellidyr eventually destroys the Crochan, at the cost of his own life.
The Black Crochan is based on a cauldron in Welsh mythology that had very similar properties.