ephialtes
an incubus; a nightmare
obsolete
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.
Where it comes from
From Ancient Greek Ἐφιάλτης (Ephiáltēs).
Adapted from the Wiktionary entry ephialtes (CC BY-SA 4.0).