|Finish the Campaign on Nightmare difficulty|
- The Achievement/Trophy name is a reference to the hardest difficulty level, "Nightmare!", in Doom, the famous first-person shooter video game also made by id Software.
- Strangely, the skull in the Achievement/Trophy image is upside-down (although this appears to be intentional rather than a goof, as blood or paint can be seen streaking downwards from the image). It also features a glowing red eye, likely a nod to the the Terminator franchise.