The Black Cauldron, virtually indistinguishable from the spirit in the Black Cauldron, is an enchanted cauldron of Dark Magic that holds the soul of a long-dead human overlord.
The Overlord was once a terrible human warlord in the British isles, and, by all appearances, a Dark Wizard to boot. So terrible and powerful was he that, faced with the threat of him, the Gods found no other way to dispose of him permanently than to seal his soul into an item of power — a large Black Cauldron, upon which the ever-frowning bearded face of the Overlord was engraved.
Although the Overlord was unable to escape his prison permanently or affect the outside world unprompted, his power still allowed him to exert influence over the world if a sacrifice of a corpse was made into the Cauldron through a particular ritual. If one as foul-hearted and powerful as he used the Cauldron so, the Overlord promised to use his power to animate an Army of the Dead, the Cauldron-Born, with which they would take over the planet and rule as gods. Once activated, the Cauldron's power could only be stopped if a living person willingly sacrificed themselves by jumping into the cauldron.
Being so dangerous, the Cauldron was taken in the care of the Witches of Morva, who kept it hidden beneath their cottage for many centuries. However, in the Dark Ages, a sorcerous warlord known as the Horned King began seeking out the Cauldron through all magical means he could devise, prompting the wizard Dallben's apprentice Taran to go on a quest to destroy the Cauldron once and for all. The foolish boy ended up revealing the Cauldron's hideaway in the process without managing to destroy it, and the Army of the Dead was risen, only to be stopped by the sacrifice of Gurgi.
Feeling its powers canceled, the Black Cauldron, mad with rage, "went out with a bang", collapsing the Horned King's Castle from the inside and destroying the Horned King's physical form, though his soul managed to escaped. Retrieved by Taran and his friends, the Cauldron was then traded back to the Witches of Morva for bringing Gurgi back to life.
The Horned King somehow regained the Cauldron by 1986 and used it to create more Cauldron-Born in a dungeon beneath Cinderella's Castle, which he had taken over with the help of other evildoers. Defeated for one final time before he just gave up altogether, the King lost the Cauldron, which ended up in the House of Mouse's prop room, guarded by the Cave of Wonders. In 1999, this led to a member of the House's staff, feather-brained Daisy Duck, trying to use the Cauldron for a completely different magical purpose (unvanishing people another spell had caused to disappear). She ended up summoning three of the Cauldron-Born instead, and then, trying to get rid of them, making the whole building vanish (an effect that Jafar was later able to cancel). Later on, Daisy defiantly tried to use the Cauldron as a fondue pot, which was apparently enough of a sacrifice of good cheese that the Cauldron again released three Cauldron-Born soldiers.
Behind the scenes
The Black Cauldron first appeared in 1985 in The Black Cauldron.
In the original Chronicles of Prydain books, the Cauldron is known as the Black Crochan and its origins are shrouded in mystery, with no hint of a thinking spirit trapped within it.
It has sometimes been assumed that the Overlord was Arawn Death-Lord, the original book series' antagonist, whose role is taken over by the Horned King in the film, although it seems more likely the Disney Horned King is simply a mix of the books' Arawn and Horned King, whose true name may in fact be Arawn.