The Cauldron-Born, or Deathless Warriors, who constitute the Army of the Dead, are or were an army of reanimated skeletons.
The Cauldron-Born are undead, invincible skeleton soldiers created in the Dark Ages by the Horned King using the Black Cauldron, using a ritual which resurrected corpses of long-dead warriors the King had gathered in his Castle. On top of retaining the combat skill of their more lively days, and literally undying loyalty to he who has risen them, the Cauldron-Born are coated in green swirls of concentrated dark magic that dissolve people into skeletal Cauldron-Born themselves if they come in close contact with it.
The Cauldron-Born have no personality or intelligence to speak of, being simply manifestations of the Cauldron's power, and "die" all over again if the Cauldron's magic is stopped. It seems that after animating them the first time, the Black Cauldron gained the ability to summon (and reanimate) a limited number of Cauldron-Born from anywhere, even should the proper ritual not be carried out (on one occasion, the Cauldron was able to summon three Cauldron-Born soldiers after being used as a fondue pot).
Behind the scenes
The Cauldron-Born first appeared in 1985 in The Black Cauldron. In the original Chronicles of Prydain books, the Cauldron-Born are not visibly skeletons, although they don't look alive either.