Welcome to the world of The Passing. It's 2007 and a surprise awaits Jonathan. The world is a decayed shadow of what it once was, and faceless creatures, The Stalkers, roam the streets. In The Passing, the protagonist, Jonathan, your typical teenager, is not a hero: he's just trying to survive, hoping that there are still some people left. It's The Passing of all life on earth. Because misery loves company
Nightmares are special events in which things will go even worse than they currently are, the Stalkers mutating and constantly screaming until the Nightmare ends. Nightmares will usually be triggered and force you to escape, fight a horde of Stalkers, fight a boss or do a puzzle, sometimes featured together (such as having to find a way to escape the Nightmare-fueled Restaurant whilst the Scissor is out to get you.
The Scissor is a stalker with two huge blades as hands, similar to the Slasher but grossly huge. It will run after Jonathan in attempt to kill him.
Nightmares can be triggered or can happen without any warning. They are recognizable by this sign flashing on the screen and the text telling you that "The Nightmare begins."
I dont think an exclemation mark sign exactly classifies as a nightmare. I'm sure the screen gets darker and everything, but the alert sign could go.