I have written plenty about The Black Cauldron over the years. This was my first project at Disney when I started there in August of 1980. The studio was finishing The Fox and the Hound, and I was asked to help design characters for the next feature film. At first it was just Tim Burton and myself, then Phil Nibbelink joined the the design unit.
After a while management decided that Tim's style would not work for this film, and that a more conventional look was in order.
I came up with the final design for The Horned King (as well as Creeper, his little henchman.)
But I did not animate any scenes with those characters. The problem was with the concept. The Horned King was just bad, that's it. In order to make your villain interesting though an only evil personality isn't enough. There need to be eccentricities, sarcasm, a weird sense of humor...or even intelligence.
Later on for future projects we would craft more rounded villains that became interesting because their personalities were richer and unique.
Ursula is the ultimate camp, Gaston is ridiculously vain and Scar really enjoys being cruel.
Actually this applies to any character in an animated film. In order to connect with an audience the main personalities have to be complex and multifaceted..like real people.