ephialtes

Etymology
From.

Noun

 * 1)  an incubus; a nightmare
 * 2) * 1662, Henry More, An Antidote Against Atheism, Book III, A Collection of Several Philosophical Writings of Dr. Henry More, p. 112:
 * "Those that were asleep it [a spirit] terrified with horrible visions; those that were waking it would strike, pull, or press, lying heavy upon them like an Ephialtes: so that there were perpetuall complaints every morning of their last nights rest through the whole Town."