Complex sentence: After their seventh relegation, in 1979, they were promoted at the first attempt; the same happened in 1984 and 1985, but after their ninth relegation, in 1986, they spent 16 years out of the top flight.
Split into simpler sentences: After their seventh relegation, in 1979, they were promoted at the first attempt. The same happened in 1984 and 1985, but after their ninth relegation, in 1986, they spent 16 years out of the top flight.