Oneiroi dark-winged spirits

The Oneiroi were the dark-winged spirits (daimones) of dreams which emerged each night like a flock of bats from their cavernous home in Erebos--the land of eternal darkness beyond the rising sun. The Oneiroi passed through one of two gates (pylai). The first of these, made of horn, was the source of the prophetic god-sent dreams, while the other, constructed of ivory, was the source of dreams which were false and without meaning. The term for nightmare was melas oneiros (black dream).

According to some the leader of the Oneiroi was Morpheus, a god who appeared in the dreams of kings in the guise of a man bearing messages from the gods.