Necromancy is the ability to raise the dead.
The Black Cauldron has the ability to raise the dead by reanimating skeletons.
Desperate to take over Prydain, the Horned King has his minions searching everywhere for the Black Cauldron. He also talks to his unanimated soldiers about telling them that he will turn them into the Cauldron-Born and ensure that they take over Prydain. He gets the Black Cauldron and throws his skeletons in and they all become the undead and the unstoppable. However, the Cauldron's power does not last forever, as Gurgi jumps in and the magic is destroyed, thus is the Cauldron'-Born's purpose.