The Nightmare Demon is a villain from the videogame, Dragon Age: A millenia old and extremely powerful demon that gladly gives the Elder One its services in exchange for the fear it feeds on.
It is responsible for the false Calling that Corypheus uses to manipulate grey wardens. Thanks to this they were ceratin that the sixth blight is comming and the world is defenseless merely 10 years after the fifth one. That made them ask magister Livius Erimond for help and he suggested to build a demon army to invade the Deep Roads and prevent the blight. In fact they were manipulated and brainwashed to Corypheus.
The Inquisition stormed Adamant fortress to stop the wardens from doing opening the veil but failed. In result the Inquisitor and companions were thrown to Fade, where they met Nightmare demon. It taunted them all, playing on their respective fears and speaking with the voice of Corypheus. Nightmare sends a fear demon to fight the Inquisitor and companions, but they win. To escape from the Fade they leave one of them to occupy the powerful demon. It's unknown what happened next.