Surprisingly dark for not only a Disney film, but for a kid's film in general. Some of the villains in the story look genuinely frightening. Like the Horned King, for example. And also, it's not every day in a family oriented movie that one of the main characters practically commits suicide like when Gergie threw himself into the black cauldron in order to stop the Horned King's undead army from destroying the world. Of course, he was brought back at the end of the movie. But still, it must have been pretty shocking for Disney to do something like this in 1985. Heck, I'd say it'd raise a few eyebrows in 2019...
All in all, a pretty good movie overall. Though I can see why it's not as revered as some of their other movies. Probably just a wee bit too dark for their target audiences at the time...