The Nightmare Man was an alien from the Seretti dimension.
History
The Nightmare man was able to pass into the universe through the nightmares of Luke Smith. (SJA: The Nightmare Man)
Abilities
The Nightmare Man was able to damage K9 Mark IV and Mr. Smith for a short while. He was also able to place those awake into a sleep, and create nightmares in the minds of those who were already asleep. (SJA: The Nightmare Man)