They tried, bless them, they really did. Disney tried to cover up the creeptastic ambiance of this animated fantasy feature with their standard big-eyed humans and cute animal characters, but it didn't work. The Black Cauldron, based loosely on the beloved Prydain fantasy novels by Lloyd Alexander, was the first animated Disney feature to earn a PG rating, thanks to its dark tone and occult themes.
And it was almost a PG-13, pushing an R-rating, until it was re-edited to remove some particularly disturbing scenes (especially for kids) involving throat-cutting, flesh-melting, and partial nudity.
The Black Cauldron is pretty standard hero's journey stuff, but the villainous Horned King and his mastery of necromancy set the movie apart from Disney's usual fantasy fare. Like Dragonslayer before it, this film lacked the typical charming princes and fairy godparents of Disney lore, favoring a darker world, in which it seemed few people had any hope for the future. The main character, Taran, is a pig-keeper leading a squalid life of obscurity, but dreams of a great destiny. His attempt to rescue his pet pig Kenwin (who produces visions in puddles of water) sets him on the path to confront one of the creepiest villains in all of Disney's filmography (voiced with icy menace by the great John Hurt).
The movie isn't considered one of Disney's better animated films (though it does have a devoted following), but it is notable as the last time the studio attempted to release a mature feature under its standard brand. After The Black Cauldron, with a couple of exceptions, Disney stuck with the bright, peppy content upon which the brand was built, and moved most of its dark content to Touchstone Pictures.
But it looks like Disney is going to take another swing at adapting Alexander's fantasy series. Let's hope it comes out better, but no less dark, than The Black Cauldron.