Nyx, Night Goddess by Gustave Moreau (1880)
In Greek mythology, Nyx is the goddess of night. Nyx literally means “Night”.
She was one of the first Protogenoi (elemental deities) to emerge from Chaos, together with Gaia, Erebus, Tartarus, and Eros.
With Erebus (Darkness) Nyx gave birth to Aether (Light) and Hemera (Day), and later – by herself – she birthed Moros (Doom), the Keres, Thanatos (Death), Hypnos (Sleep), the Oneiroi (Dreams), Momus (Criticism), Oizys (Misery), the Hesperides, the Moirai (the Fates), Nemesis (Retribution), Apate (Deceit), Philotes (Friendship, Love), Geras (Old Age), and Eris (Strife).The Underworld is – according to Hesiod – the home of Nyx and the home of her children Hemera, Hypnos, and Thanatos. Hesiod says further that Hemera leaves the Underworld just as Nyx enters it, and when Hemera returns, Nyx leaves.
Hesiod, Theogony 744 ff (trans. Evelyn-White) (Greek epic C8th or C7th B.C.):
“And there [at the ends of the earth, where sky meets earth], all in their order, are the sources and ends of gloomy earth and misty Tartaros (…) There stands the awful home of murky Nyx (Night) wrapped in dark clouds. (…) Nyx (Night) and Hemera (Day) draw near and greet one another as they pass the great threshold of bronze: and while the one is about to go down into the house, the other one comes out the door. And the house never holds them both within; but always one is without the house passing over the earth, while the other stays at home and waits until the time for her journeying come; (…)”
When Nyx leaves the Underworld, she brings Erebus with her, and places him on the sky, causing the world to darken, until Hemera comes out and scatters the dark mist of night.