The first game was originally going to end with Chernabog (as either THE darkness or a personification of the Darkness) attacking Sora. Sora would fight his way up Bald Mountain and then the light from Kingdom Hearts would vanquish Chernabog like how the sun/God defeated him in Fantasia.
Lack of time, memory, and system power kind of squashed that idea. Which is a pity, it sounds AMAZING. However, they HAD finished the Chernabog model so it got thrown in as a boss fight in the last world. But, you just flew around and attacked. It was still cool, primarily because of this:
They played Chernabog's song.
