After the deaths of both Walt and Roy Disney, the movie studio they founded was in a bit of flux. They couldn't determine where film trends were going, and the wholesome live-action movies they produced in the early 1970s didn't see the success that type of movie had in the '60s. Then George Lucas approached Disney about his project called Star Wars, and they turned him down, because it appeared to be science fiction, and everyone knew that wouldn't fly. At the same time, studio executives wanted to court the lucrative teen market. That led them to produce a string of darker, more dangerous films over the next few years.
To this end, Disney began work on the most expensive movie in its history: The Black Hole. Work on the sci-fi adventure had begun in the mid-70s, where it began as an unpublished story called Space Station-One. For years, the project floated around Disney's offices, with writers coming and going and the name changing first to Probe One and then finally to The Black Hole. It wasn't until January 1978 that pre-production began in earnest, and by this point, Star Wars had punched a planet-sized hole through public consciousness, and sci-fi was suddenly the hot genre.
The Black Hole's then-huge $20 million budget wasn't the only precedent the movie would set: it was to be the first Disney production to carry a PG rating. This might not sound like a big deal today, but at the time, it was quite a departure; traditionally, Disney had a policy of only releasing G-rated movies, which it stuck to rigidly - when the company's 1950 film Treasure Island was reissued in 1975, a brief shot of a bullet wound was snipped out to avoid a PG rating from the MPAA.
Other movies followed: The Watcher In The Woods, Dragonslayer, The Devil And Max Devlin, Tron, and others. They were such a departure from the House of Mouse we knew, that they don't even seem like Disney films now. Read about what Disney was thinking with each of them at Den of Geek.