Epialtes dicunt greci nos effialtes incubus.
nos ABC fjp | nomĩe ms. e
effialtes AC ef | efialtes B | efficaltes jp
Ms. p adds an attempts at writing ἐπιάλτης /epiáltēs/ in Greek script
Epialtes is what the Greeks say, we say effialtes or incubus.
Cf. Greek ἐφιάλτης /ephiáltēs/, and the Aeolian form ἐπιάλτης /epiáltēs/, means "nightmare, conceived as a throttling demon" (LSJ). Why Simon sees the Aeolian form as the common Greek form remains unclear. The word is of uncertain etymology but even in antiquity it was folk-etymologically connected with ἐφάλλομαι /ephállomai/ "to spring upon, attack".
The incubus was believed to be a male demon that "lies upon" – Latin incubo – people, normally women, in their sleep for sexual gratification. The female of the species is called succubus, because she normally "lies under" a male sleeper – Latin succubo "to lie under" - to steal intercourse. To the medieval mind no good could come of it and it was thought to lead to the deterioration of the victim's health and ultimately to death. Night demons with sexual intentions are a staple of most ancient mythologies.
See also: Effialte