'Fantasia' is a strange movie in a number of ways - it's very dark by the standards of a Disney animation, and Chernabog is perhaps the most evil and terrifying Disney creation of all time.
You only have to look at Chernabog once to suffer from nightmares as a child. The demonic beast is almost the manifestation of Satan himself, and he brings a whole dark army with him in his wake. When he appears in the film, it's like the gates of Hell have opened.