In the book series, he appears only in the first book, The Book of Three. He is Arawn's champion and the War Leader of Annuvin. The Horned King is described as a huge man wearing armor (except for his arms, which are naked and stained crimson), a red cape and an antlered skull mask. The antlers are presumably where he gets his name. Arawn sends him to capture the pig Hen Wen from Caer Dallben. He followed Taran across Prydain in search of the oracular pig Hen Wen, but could not find her, and attacked Taran at Caer Dathyl, trying to gain the pig. Taran tries to fight him, using the sword Dyrnwyn, but is nearly slain by the sword's fire in his attempt to draw it. Gwydion is able to defeat the Horned King by speaking his true name, which is never revealed.
The second book concerns itself with Arawn's desire to possess the Black Cauldron. The film The Black Cauldron combines scenes from the first two books, and uses the Horned King in place of Arawn. He looks quite different in the movie; the antlers remain, but he wears a red hooded robe. Instead of a normal human, he appears to be an undead, skeletal creature (a lich). In the movie, he searches for the title object, seeking to use its power to bring to life an army of undead soldiers and with them, conquer the world. Deploying his servant, Creeper, the dragon-like Gwythaints and human soldiers, the King seeks out the pig Hen Wen to aid in his quest. He eventually accomplishes his goal, but is thwarted some time later, when Gurgi throws himself into the Cauldron. When he attempts to cast Taran into the Cauldron as well, he ends up being consumed by the Cauldron's power and destroyed. Although the movie was a failure at the box office, John Hurt's voice was considered one of the better performances and the Horned King is widely regarded to be one of Disney's best and most sinister villains.