The Black Cauldron is the title object of The Black Cauldron. The Horned King seeks it in order to become immortal and to dominate the world. Fueled by the spirit of the dark lord Arawn, the Black Cauldron was nigh-indestructable.
With the Black Cauldron, the Horned King was able to call up the monstrous undead army of the Cauldron Born. With these forces, he could conquer all of Prydain. However, all of the Black Cauldron's evil powers could be undone if a soul willingly climbed into the cauldron at the cost of their own life, which Gurgi chooses to do to save his friends. The Black Cauldron was held in Morva for centuries until Taran and his friends sought it out to try and destroy it.
Role in the series
The Black Cauldron reprises its role from the movie, only it now has the added function of being a little-used portal to Hell and the dimensions where Chernabog's forces lie. Apparently, after Arawn was thrown into the Cauldron, Chernabog refused to let the prison of his most successful warlord be wasted. So, he placed a heavy enchantment on the item, giving it the ability to resurrect the dead and make them cauldron-born, and serve as a portal to Hell for his various darkspawn forces during the first Cartoonian War...just because there weren't enough ways for the Darkspawn to come into our world. Truth to say, the Black Cauldron was found by the Horned King, who used both of its functions to try and become a god amongst the Darkspawn, and serve as the new successor of Chernabog instead of that dragon, Malice or something. Oh yeah, and there was this Rasputin like snake that looked like it was made by a computer game that the Horned King had promised to restore to its full power with the heart of the Princess Kairi, but he was only lying cause Cobra is a darkness-addicted fool consumed by thoughts of only corruption not worthy for the Horned King's time. Unfotunately, the cauldron's summoning of the demons left them weak and without most of their powers. All they did was laugh evil and stay behind in the throne room, because they felt the universe was not yet ready for their arrival. Malefor, the real name of Chernabog's successor, in particular, was digusted by his minions' stupidity for their wasted potential, but felt that first he would have to get rid of Spyro before setting out to conquer the universe. Of course, the cauldron's powers were resevred by Gurgi, and well you know the rest.