The Cauldron Born are skeletons wearing many varieties of armor and weapons.
Eventually, they are dark, scary, and pure evil. Their goal is to destroy all living beings in their path.
As some of the more nightmarish beings in Disney media, considering their undead zombie nature, many of the deleted scenes of the film surround the waking of the Cauldron Born and their attacks. One of the more infamous cuts and one that an image of would find its way online of would be a scene of the green mist from the cauldron melting the skin off the bones of one of his mortal soldiers.
The primary reason why the Horned King is searching for the cauldron, he is often seen speaking to the dead bodies littering his castle. When the Cauldron finally gets into the Horned King's grasp, he calls upon it's terrible power to awaken the corpses as the army of the Cauldron Born, who proceed to kill all of his mortal minions on their way out of the castle and across the bridge into Prydain. However, they are destroyed when Gurgi sacrifices himself to end the Cauldron's power. In disbelief of his deathless warriors returning to piles of bones, the Horned King attacks Taran before being sucked into the cauldron and destroyed.
The skeletons are then obliterated after the castle explodes.
The Cauldron Born appeared at the end of the Cinderella Castle Mystery Tour at Tokyo Disneyland, where the Horned King brought them to life to attack the guests. However, they were defeated alongside their master when a guest was chosen to use a magic sword to destroy the Cauldron and beat the Horned King with it.