nightmare or demon that causes nightmares, c.1600, from Greek Ephialtes, name of a demon supposed to cause nightmares; the ancient explanation is that it was from ephallesthai "to leap upon," but OED finds "considerable" phonological difficulties with this.
Examples from the Web for ephialtes
But the former was the home of Ephialtes and Otus, and the latter was the dwelling-place of Orion.Plutarch's Morals
The detachment which Ephialtes had guided were coming up from below.Xerxes
The scene was closed by the arrival of the Persians led by Ephialtes in the rear.Historical Parallels, vol 2 of 3)
Arthur Thomas Malkin
The privileges of the Areopagus were Cleisthenes, Ephialtes.
She bore two sons Otus and Ephialtes, but both were short lived.The Odyssey