Disney had, in 1971, acquired the rights to Lloyd Alexander's "The Chronicles of Prydain". The movie, which took over twelve years to make, is loosely based upon the first two books ("The Book of Three" and "The Black Cauldron"). The Chronicles, in turn, are loosely based on the mythology of ancient Wales, a collection of tales known as the Mabinogion. Because of the numerous storylines and with over thirty characters in the original five-novel "Chronicles of Prydain" series, several story artists and animators worked on the development of this movie throughout the 1970s, where it was actually originally slated for release in 1980. The release date was eventually pushed from 1980 to Christmas 1984, in order to focus more attention on the completion of The Fox and the Hound (1981), and also due to the animators' inability of animating realistic human characters for this movie.
The Black Cauldron represented Walt Disney Productions' attempt to reach out to teenage fans of fantasy novels, a popular genre at the time. Test audiences left the theater in droves, however, which caused the studio to cut 12 minutes of the most disturbing imagery from the original film. It had also reignited debates about whether or not the animation movie genre can appeal beyond the children audiences, in light of the movie's dark, graphic nature; Jeffrey Katzenberg's controversial decision to edit and re-work the majority of the movie, when it was already completed prior to his arrival at the Walt Disney Studios; and the growing concern that mature audiences had no interest in sitting through and watching an animated movie, let alone any movie that was produced by Walt Disney Studios.
Info compiled from the IMDB Trivia page for the film