The Nightmare shows a woman in deep sleep with a demonic creature crouched on her chest. The work was likely inspired by Germanic tales of demons and witches that possessed people who slept alone. Men were visited by horses or hags, and women were believed to engage in sex with the devil.
The impish creature on her chest is an incubus. An ape-like demon who lies upon sleeping women in order to engage in sexual activity with them.
The work was scandalous due to it’s sexual themes and was thought to inspire a scene in Mary Shelley’s Frankenstein: “She was there, lifeless and inanimate, thrown across the bed, her head hanging down, and her pale and distorted features half covered by hair.”