Enter the Goths
The European Enlightenment of the 18th Century praised logic and reason over superstition and mysticism. Fantasy was kicked to the literary curb for a while, but people clearly missed it. It wasn't long before the Romantic generation was rebelling against the idea that logic ruled all, and in the process they pioneered a morbid underbelly of fantastic literature. In 1764, Horace Walpole published the first gothic novel, The Castle of Otranto. People went absolutely mental over it.
Walpole's popularity inspired a tidal wave of dark meditations on death, devilry, and the supernatural that is still thriving today. Carried by the tide were thinkers like Mary Shelley, Edgar Allen Poe, Bram Stoker, Flannery O'Connor, Neil Gaiman, and Tim Burton.
Vader was right: never underestimate the power of the Dark Side.