The Oneiroi were born in the first darkness, the children of Nyx, the primordial Night, who gave birth to them without a father in the yawning void before the making of the world. In Hesiod's Theogony, the great poem of divine genealogy composed around 700 BCE, they are listed among the children of Night alongside the Fates, the Furies, Nemesis, and Death, a family of powers that govern the aspects of existence no mortal can escape. But in the Homeric tradition, they are the sons of Hypnos, the god of sleep, who dwells in a cave near the river Oceanus where the sun never shines and where the poppies and other sleep-inducing flowers grow in profusion. Hypnos is their father and their master, and he sends them forth each night like a king dispatching messengers to every corner of his realm.

The cave of Hypnos is described in the Odyssey, the epic of wandering and return composed around the eighth century BCE, as a place of absolute darkness and silence, where no bird sings and no wave breaks upon the shore. The entrance is framed by poplars and willows, trees whose drooping branches seem to weep with drowsiness, and the floor is thick with the petals of opium poppies, the flowers that bring forgetfulness. Hypnos himself lies upon a bed of black ebony, covered with dark feathers, and around him flutter the Oneiroi, their black wings beating softly in the still air like the wings of bats in a cavern. When night falls over the world, Hypnos opens his eyes and sends them forth, one for every sleeper, one for every dream.

The most famous description of the Oneiroi in action comes from the end of the Odyssey, when Penelope, the queen of Ithaca, has waited twenty years for her husband Odysseus to return from Troy. A dream-spirit comes to her in the shape of her sister Iphthime, or in the shape of a woman she trusts, and speaks words of comfort that are both true and false, for the Oneiroi are not simple messengers but complex deceivers who mix truth with lies as a painter mixes colours on a palette. The spirit tells Penelope that Odysseus is near and that her grief will soon end, and Penelope wakes with a heart lighter than it has been in years, though she does not yet know that the dream was sent by Athena, the goddess of wisdom, who guides the Oneiroi as she guides all things that serve her favourites.

The gates through which the Oneiroi pass are described in the Odyssey as well, in the passage where Penelope speaks to the disguised Odysseus about the nature of dreams. There are two gates, she says: one made of horn and one made of ivory. The dreams that pass through the gate of horn are true, the visions that warn of danger, that prophesy the future, that bring messages from the gods. The dreams that pass through the gate of ivory are false, the visions that deceive and torment, that show us what we desire and then snatch it away when we wake. The gate of horn is hard and translucent, like the substance of truth itself, while the gate of ivory is smooth and white and beautiful, like the surface of a lie that conceals the darkness beneath. Every dream that comes to a sleeper has passed through one of these gates, and the art of the wise is to know which gate their dream has used.

The Oneiroi were not merely passive messengers. They could be sent by the gods to punish as well as to comfort. When Clytemnestra, the queen of Mycenae, murdered her husband Agamemnon on his return from Troy, the Furies, the avenging spirits who punish murder within the family, sent terrible dreams to haunt her sleep, dreams of blood and pursuit and the gaping mouths of the dead. The Oneiroi carried these visions, for the Furies themselves do not enter the bedrooms of mortals but send their servants to do their work. When Orestes, the son of Agamemnon, killed his mother to avenge his father, the Furies pursued him with dreams so terrible that he wandered Greece in madness, and the Oneiroi were the wings that carried those dreams to his pillow every night.

The Greeks did not worship the Oneiroi with temples or sacrifices, for they were spirits rather than gods, powers of the night that could not be petitioned or bargained with. But they acknowledged them in their daily speech, calling a beautiful dream oneiros and a nightmare ephialtes, the leaper-upon, the spirit that sits upon the chest and brings paralysis and terror. The physician Hippocrates, the father of medicine, wrote that dreams were sent by the body itself, the Oneiroi emerging from the humours and the organs rather than from the underworld, but even he used the old names, for the Greeks could not imagine sleep without its winged visitors. To the end of antiquity, the Oneiroi remained the explanation for the mystery that every human being experiences and none can fully explain: the world that opens when the eyes close, and the visitors who come to us there.