ALASTOR was the daimon (spirit) of family blood feuds and the inflicton of vengeance upon a new generation for the crimes of their fathers. He was related to the Erinyes, the avengers of murder, although the retaliation which Alastor inspired was directed against the murderer's family rather than against the murderer himself.
Probably NYX though nowhere stated
Aeschylus, Agamemnon 1468 ff (trans. Weir Smyth) (Greek tragedy C5th B.C.) :
"Chorus [of Argive elders] O Daimon [i.e. Alastor] who falls upon this house and Tantalos' two descendants [Agamemnon and Menelaus], you who by the hands of women [i.e. their wives Klytaimestra and Helene] exert a rule matching their temper, a rule bitter to my soul! Perched over his body like a hateful raven, in hoarse notes she chants her song of triumph.
Klytaimestra (Clytemnestra): Now you have corrected the judgment of your lips in that you name the thrice-gorged Daimon of this race [presumably this refers to the murders of the sons of Thyestes, of Atreus, and of Agamemnon]. For by him the lust for lapping blood is fostered in the mouth; so before the ancient wound is healed, fresh blood is spilled.
Truly you speak of a mighty Daimon (Fiend), haunting the house, and heavy in his wrath (alas, alas!)--an evil tale of catastrophic fate insatiate; woe, woe, done by will of Zeus, author of all, worker of all! For what is brought to pass for mortal men save by will of Zeus? What herein is not wrought of god? Alas, alas, my King, my King, how shall I bewail you? How voice my heartfelt love for you? To lie in this spider's web, breathing forth your life in an impious death! Ah me, to lie on this ignoble bed, struck down in treacherous death wrought by a weapon of double edge wielded by the hand of your own wife!
Do you affirm this deed is mine? Do not imagine that I am Agamemnon's spouse. A phantom resembling that corpse's wife, the ancient bitter evil spirit (alastôr) of Atreus, that grim banqueter, has offered him in payment, sacrificing a full-grown victim in vengeance for those slain babes [the murdered sons of Thyestes].
That you are innocent of this murder--who will bear you witness? How could anyone do so? And yet the evil genius (alastôr) of his father might well be your accomplice. By force amid streams of kindred blood black Ares (Havoc) presses on to where he shall grant vengeance for the gore of children served for meat [i.e. when Atreus served the sons of Thyestes to their father as a meal]."
Aeschylus, Agamemnon 1560 ff :
"Chorus [of Argive elders]: Reproach thus meets reproach in turn [in the blood feud of the house of Atreus]--hard is the struggle to decide. The spoiler is despoiled, the slayer pays penalty. Yet, while Zeus remains on his throne, it remains true that to him who does it shall be done; for it is law. Who can cast from out the house the seed of the curse? The race is bound fast in calamity.
Klytaimestra: pon this divine deliverance have you rightly touched. As for me, however, I am willing to make a sworn compact with the Daimon [Alastor] of the house of Pleisthenes [i.e. the Atreides] that I will be content with what is done [the murder of Agamemnon], hard to endure though it is. Henceforth he [Alastor] shall leave this house and bring tribulation upon some other race by murder of kin. A small part of the wealth is fully enough for me, if I may but rid these halls of the frenzy of mutual murder."
- Aeschylus, Agamemnon - Greek Tragedy C5th B.C.