Nyx goddess of night, in Greek mythology, was so powerful, that everyone feared her, including Zeus. In the Illiad, Homer says that she "has power over gods and men".
She was one of the primeval deities, born from Chaos, a sister of Erebus (Darkness), whom she married. From their love several children were born: Aether (Atmosphere), Hemera (Day), Hypnos (Sleep), Thanatos (Death), Nemesis (Retribution), Moros (Doom) and maybe Styx. (I really find it curious that from two divinities of the darkness, the light was born). By parthenogenesis, she had Eris (Strife), the Hesperides, Oneiroi (the Dreams), the Moirae (the Fates), Geras (Old Age, whom she bore so as he could afflict people.
Nyx goddess of night was very beautiful and very powerful. In the Illiad, Hera asks Hypnos to help her, by making Zeus sleep, so that she should have time to interfere with the war behind his back. But Hypnos turns her down, because the last time he had helped Hera, Zeus found out and Hypnos had to take refuge near his mother, Nyx, in order to get rid of Zeus' punishment. Nyx would confront Zeus with a "maternal fury", so that's why even the king of all gods avoided stepping on her toes!
She was represented as a winged goddess and was said to wear a veil of black mist on her head. In other descriptions she wears a "spangled robe" or a "blue-black headband" or a "blue-black robe". She was also described as a charioteer, who guides her chariot accross the sky at night. In Ovid, she is described as having a poppy garland on her forehead.
Some mentions about Nyx goddess of night come from descriptions of ancient paintings. Philostratus the Elder describes a scene in which Eos was mourning the death of her son, Memnon. Her tears made Helios go away and made Nyx cover in darkness the army, so that Eos could steal her son's body.
Philostratus the Younger describes a painting in which Herakles, as a baby, strangles the two serpents sent to kill him. Nyx is present too, with a torch, so as to shed light on this scene, so that everyone could see how valiant the little child was.