They lived in caves in the far west, near the gates of the Underworld. They were sent through one of two gates there. One gate was made of horn, from there the true or prophetic dreams come from. The other gate was made of ivory, from there the false and meaningless dreams come from.
They were shown as black-winged daemons.
The most powerful of the Oneiroi was Morpheus. His brothers Phobetor and Phantasos shape parts of dreams, while Morpheus shapes the dream in general. Morpheus shapes human figures, Phobetor shapes animal figures and Phantasos shapes inanimate objects. Morpheus is still depicted in popular fiction today.
Oneiroi Facts for Kids. Kiddle Encyclopedia.