I thought the Envy demon in Champions of the Just was more interesting than the Nightmare. Nightmare just uses some dumb taunts that most everyone shrugs at, goes ~mwahahaha I am evil fear me~ a lot, generally comes across as a narmy cartoon villain. "I grow fat on your fear" was some quality level of "this hurts you, Shepard" cheese.
Envy was a lot more effective at creeping into the Inquisitor's fears and unnerving them, basically by playing up the fear of the self. Instead of some dumb taunts about how you will fail because I am so evil and strong blah blah blah, Envy taunts you by showing how it will lead the Inquisition to ruin using your face, how ultimately you will be the villain of the story, the name everyone reviles as destroyer of worlds, etc. That's a much more personal threat, the stakes are not just "defeat the big spider" but "save yourself from yourself." Just the idea of how the world would turn out that's on the death slide in that quest is terrifying, your advisors all smiling and trusting Envy because it wears your face... pity the Haven illusion part of that quest was cut, it would have been top creepy.