EPIALES was the personified spirit (daimon) of nightmares. He was also known as the melas oneiros "black dream". Epiales was probably numbered amongst the Oneiroi (Dream-Spirits), sons of the goddess Nyx (Night).
The Greek word epialês was derived from the verb epiallô "to lay upon."
Probably NYX, though nowhere stated
CLASSICAL LITERATURE QUOTES
Greek Lyric I Alcaeus, Frag 406 (from Etymologicum Genuinum 151) :
"The words epialos, epiales and epioles denote (1) the feverish chill (2) the daimon who assaults sleepers. Homer and most writers have epioles with the e; the form in -os means something different, namely the feverish chill . . . Alkaios (Alcaeus) called it epialos. Apollonios says that Epialtes itself (the nighmare daimon) is called Epiales and by a change of a to o Epioles."
Aeschylus, Suppliant Women 886 ff (trans. Weir Smyth) (Greek tragedy C5th B.C.) :
"[The goddess Gaia (Earth) is invoked to drive away a nightmare :] Like a spider, he [a rapist] is carrying me [a woman] seaward step by step--a nightmare (oneiros), a black nightmare (melas oneiros)! Oh! Oh! Mother Earth (Ma Ge), mother Earth (Ma Ge), avert his fearful cries! O father Zeus, son of Ge (Earth)!"
- Greek Lyric I Alcaeus, Fragments - Greek Lyric C6th B.C.
- Aeschylus, Suppliant Women - Greek Tragedy C5th B.C.
Other references not currently quoted here: Phrynincus Comicus 1, Dioscorides 3.140, Artemidorus 2.37, Rufus ap. Orib. 7.26.177, Strabo 1.2.8.