The Oneiroi

"Pray well and often for sweet dreams from The Oneiroi, whose black wings soar across the Dream Realm."

Sometimes labelled as messenger angels, more often called demonic servants, The Dream Lords are actually minor underworld deities, similar to The Furies - though weaker and more numerous.
These lesser gods rule the Dream World, appearing in people's mind to construct every dream they have. Masters of shapeshifting and realm shaping, they are the characters and monsters, landscapes and buildings that make up the world people visit when they sleep.
Such is their power that Creation's true, Greater Gods reach out to them whenever they need to deliver an omen or vision to their followers.

Gates of Horn and Ivory

It is said there are two gates through which The Oneiroi can reach out to affect the minds of mortals. Whensoever they travel through the Gate of Horn, the dreams they grant are true; old memories replayed anew, or prophetic visions granted by higher beings. But when they reach through Ivory, the hearts of mortal shall be deceived.

Kýeri, The Dream Tyrant

"Beware the Father of Nightmares, whose reach extends though either Gate and yet delivers terror all the same."

Aeons ago, one of the most powerful Oneiroi began to reject his orders and purpose in the Cosmos, turning way from delivering pleasant dreams and messages for the Gods. He began shaping darker dreams, nightmares to haunt people's resting hours and foul their moods even once they awaken. Some claim he does this out of spite, others believe he harvests a little piece of a person's soul each time they have such a nightmare, growing evermore powerful.
As with all gods he has many names and titles, though his most common are The Dream Tyrant and The Nightmare King.