The Nightmare is a fear demon of unparalleled size and power.
—From Codex entry: Nightmare (Inquisition)
After Corypheus fails to acquire the anchor to pass through the Fade and reach the Black City, Nightmare reaches out to the Herald of Andraste's mind in the Fade and magically seals their memories of the events at the conclave to prevent them from revealing what they discovered.
At Corypheus' behest, Nightmare helps deceive the Grey Wardens of Orlais into thinking they are all hearing the Calling simultaneously. With the Grey Wardens convinced they are about to be wiped out and Thedas left defenseless against the inevitable Sixth Blight, they turn to magister Livius Erimond who convinces them they can storm the Deep Roads with an army of bound demons and kill the remaining Old gods before they become Archdemons, thus preventing any future Blights. In truth however, the ritual will also make the summoning mages thralls of Corypheus. The Inquisition assaults Adamant Fortress just as Warden Commander Clarel is about to bring Nightmare through the Veil. The Grey Wardens and the Inquisition turn on Erimond when the truth is revealed. However Corypheus' red lyrium dragon destroys the walkway the Inquisitor, Hawke and their companions are standing on just as Clarel drives off the dragon and to save themselves the Inquisitor opens a Fade rift to escape the fall into the chasm below.The Inquisitor and their allies find themselves trapped in Nightmare's section of the Fade. As Nightmare notices their presence, it begins to taunt the party of intruders one by one, playing on their respective fears and speaking with the voice of Corypheus. Nightmare also inadvertently reveals that it is the key to Corypheus' demon army and without Nightmare, the bound demons will fail. The Inquisitor and their allies finally confront Nightmare in person as they reach the rift back into Thedas. A spirit or perhaps, the soul of the late Divine Justinia V, attacks Nightmare to allow the party to escape, scattering Nightmare's corporeal form. However though Nightmare itself is weakened, a separate Fear Demon which is part of Nightmare's self, the Aspect of Nightmare, escapes unharmed and prevents the party's escape. The Aspect is finally slain, but only half of the party are able to escape before Nightmare re-forms and blocks the passage once more, leaving the Inquisitor, Hawke and their Grey Warden ally trapped in the Fade. Either Hawke or the Warden ally sacrifice themselves to distract Nightmare long enough for the Inquisitor and the other companion to escape back through the rift, closing it behind them and leaving Nightmare sealed in the Fade.
- (To Blackwall, regarding his deepest fear) "Ah, there's nothing like a Grey Warden. And you are nothing like a Grey Warden."
- (To Cole, regarding his deepest fear) "Are you afraid, Cole? I can help you forget. Just like you help other people. We're so very much alike, you and I."
- (To Solas, regarding his deepest fear) "Dirth ma, harellan. Ma banal enasalin. Mar solas ena mar din."
- (To Iron Bull, regarding his deepest fear) "The Qunari will make a lovely host for one of my minions. Or maybe I will ride his body myself."
- (To Vivienne, regarding her deepest fear) "What's it like living as an apostate, Vivienne? Do you really think you'll reclaim your power in the circle... at your age?"
- (To Dorian, regarding his deepest fear) "Greetings, Dorian... It is Dorian, isn't it? For a moment, I mistook you for your father."
- (To Cassandra, regarding her deepest fear) "Your Inquisitor is a fraud, Cassandra. Yet more evidence there is no Maker, that all your 'faith' has been for naught."
- (To Sera, regarding her deepest fear) "Sera. Sera. Sera... If you shoot an arrow at me, I'll know where you are."
- (To Varric, regarding his deepest fear) "Once again, Hawke is in danger because of you, Varric. You found the red lyrium. You brought Hawke here..."
- (To Hawke, regarding their deepest fear) "Did you think you mattered, Hawke? Did you think anything you ever did mattered? You couldn't even save your city. How could you expect to strike down a god?"
- According to Cole and the spirit encountered in his lair, this demon originally started off with the desire to help: taking fear away so people could move on. Over time, its desire to consume fear and power twisted it into what it became.
- It appears that the demon has a large knowledge of Elvish, as shown when it speaks to Solas.
- The demon has unique dialogue for each companion and every possible Warden ally regarding their deepest fear or secret.
- In the game files, this creature is called Grand Fear Demon, abbreviated to GFD.
- David Stern, who is the voice actor for the Nightmare, is also the voice of Corypheus.