The Oneroi are Dream-Hunters responsible for watching over human dreams and protecting them from Skoti. They live on the Vanishing Isle, where they have portals used to monitor human sleep and join the sleepers. While the Oneroi can technically do their job without these portals, they make it easier and more effective. The Vanishing Isle also holds the Onethalamos, which is the room where the three leaders of the dream gods make policy. It's a fairly large conference room.
Oneroi are often assigned to newly created Dark-Hunters to heal them mentally. Since the Greek pantheon derives its power from human worshipers, many Greek gods lost power as their popularity waned. However, the Oneroi maintained the strength of their powers through the sheer number of Dark-Hunters needing their help.