Nightmare is a Debuff that is inflicted by monsters in Nightmare Mode.
When inflicted, the sides of the screen will turn red, and the screen will wobble. The presence of nightmare creatures are seen on the side of the screen. It lasts for 15 seconds.
- The effects of the debuff and the presence of nightmare creatures is a reference to the game Don't Starve, in which when you go insane, the nightmare creatures will attack you.