The 1st Nightmare
The Nightmare God is old. Birthed when Creation made sleep and from it, nightmares. It is a living concept that exists to terrorize the subconscious mind since that was it’s purpose.
Shortly after it was formed, the Nightmare God was imprisoned between the farthest layers of the dream realms. A massive filter was put in place to keep the unfinished dreams and half thought fantasies from spilling into the outer realms from one end and block the influence of the Nightmare God from driving all of the sentient races mad in their sleep from the other. This is the only physical wall of it’s prison. The rest is unending blackness. Despite all of this, the gods power is too massive to block out completely. It’s influence seeps through enough that those who slumber and dream can still experience horrors.
It very nearly escaped once, owing mostly to the gods first acolyte The Old Man on the Throne of Screams. Thankfully it was stopped and the god remains in its prison.