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 the Horned King to capture the pig Hen Wen from Caer Dallben. He follows Taran across Prydain in search of the oracular pig Hen Wen, but cannot find her. In the end, he marches his army to Caer Dathyl and attacks Taran when he and Eilonwy try to warn the Sons of Don of the impending attack. Taran tries to fight him with 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; upon the utterance of his name, the Horned King is struck by a lightning blast and consumed by flames.
The animated Disney film The Black Cauldron combines scenes from the first two books, and uses the Horned King in place of Arawn as the movie's prime antagonist. 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 whose eyes glow red upon being angered or excited (a lich) and is more on par with a sorcerer. He was voiced by John Hurt.
In the movie, the Horned King searches for the title object, seeking to use its power to bring to life an army of undead soldiers and 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.
In addition, the Horned King makes appearances in several of Disney's House of Mouse.
- In one episode, he enters the club and shakes hands with Donald.
- In another episode, he sits at a table and laughs with other characters after seeing Goofy fall into a gopher hole dug by Gopher.
- In another episode, while Goofy sings a song that's a spoof of "Supercalifragilisticexpialidocious!", he is seen at a table where a penguin waiter serves him his dinner.
This same version appeared as the final boss in the Sega Master System and Game Gear title, Land of Illusion starring Mickey Mouse. The Horned King goes under the moniker of "The Phantom" in the game and is colored differently, white bones and black robe as opposed to green bones and crimson robe.
In Italian version Horned King was called Re Cornelius (King Cornelius) because the Italian word for horned (cornuto) means also "a person that has been betrayed by the proper partner".