EPIALES was the spirit (daimon) of nightmares, also known as melas oneiros (Black Dream). The Oneiroi or Dream Spirits were sons of the goddess Nyx (Night). The word epialês was ultimately derived from the verb epiallô, "to lay upon."
Probably NYX like the Oneiroi, though nowhere stated
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.) :
"[In the following passage Gaia, goddess of the 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