- Icarus. Everything about Icarus. From his insanely scary voice to him just about stalking you all through Paris, it's one gallon of nightmare fuel after another.
I installed UNATCO's security myself, it's unbeatable! You're starting to scare me...
- He first appears when you're down in a sewer. MJ12 soldiers and their robots are right above you, and they're looking for your head. You pass on, and suddenly, your infolink rings, revealing a demonic voice and a creepy looking eyeball. "I now have access to your systems."
- Answer a ringing phone in an empty office building, and the voice on the other end (Icarus) will tell you that he is right behind you, and will soon be "a part of everything in your world." Check it out here.
- Take the time to look at a Paris public terminal and you'll find an entry titled "READ ME." Take a look at it and you'll find the phrases "ICARUS FOUND YOU!!!" and "RUN WHILE YOU CAN!!!" repeated over and over, in a > shape.
- While going through the DuClaire mansion, he informs you that she's been executed and dissected.
- Daedalus, the more benevolent counterpart of Icarus, inadvertently has qualities like this, since his first contact with you (in the Majestic 12 cell) comes right out of the blue, and you won't learn his/her/its true goals until much later in the game. Alex Jacobson adds a cherry to the top by noting:
- Also, freaking greasels. They're weird little chupacabra-looking creatures that spit acid. They're tiny compared to you, so you never know if you'll run into one in a cramped ventilation shaft. Also, they make this really creepy noise whenever you're near them. To top it all off, they're very hard to kill.
JC: Bravery is not a function of firepower.
- Greasels are like this In-Universe as well - you can read the datacubes and e-mails of scientists throughout the game, and they are 100% correct in being wary of them. There's also Aimee, the Parisian woman distraught over losing her cats to greasels...while she is understandably afraid of them, she has also noticed that even the Majestic 12 Commandos are afraid of them.
- The real nightmare fuel in this game comes from an unexpected source: cats. They don't look very scary, wandering around streets and houses. But if you accidentally step on one, they let out a horrendous hissing screech that will cause you to jump out of your chair. If you step on them again, you kill them.
- The spider bots. No, not the normal ones, they're just annoying; I mean the large model that is taller than JC. Mercifully there are only two or three of them in the game.
- The conclusion to a log found in Chow's secret lair:
YK: And you... you... are a bad actress.MC: And you have one less finger...
- The notoriously well-hidden secret area in the Hong Kong Canals - a broken tunnel under the VersaLife building. In it, you find Karkians, a Universal Constructor, the body of a dead man, and his last written words. Doubles as a Tear Jerker:
...God's got a funny sense of humor sometimes... I'm writing this on the back of an issue of "Tomorrow's Scientist."Hundley sent me to pick up a batch of karkian eggs fresh off the plane; couldn't trust anyone else, Dr. Feng — that's what he said. Big job. On the way back through the tunnel everything just fell apart, flash of light and noise and suddenly I'm lying here. My legs are broken. It was probably a Triad attack... they've been fighting lately...I thought emergency services would arrive, but no one's come. There's no internal bleeding that I can tell. I started to hear noises a while ago. I think the suspension crate in the car broke open and the eggs are hatching. I can't reach anyone on my phone. It's been eight hours. I slept a little. I can hear the baby karkians — cries of hunger, precursor to infant food seeking behavior. The irony is this is a perfect environment for them.I saw a shadow of one a little ago. My rough guess is they're developing almost 20% more quickly than expected. Some nasty noises earlier, several of the karkians picking out the weakest of the litter and tearing it to pieces.
- The world of Deus Ex is described by Word of God as "five minutes before the fall of human civilization." and it shows. Massive resource shortages, natural disasters, colossal class divisions, unchecked corporate power, government "incompetence", rampant terrorism, a plague decimating the lower classes and the increasingly brutal and invasive suppression of unrest have put human society on the brink of complete collapse. The game's many decaying and desolate settings paint a grim picture, and things only go From Bad to Worse as JC gets closer to the truth of what's happening. It doesn't help that several plot points would be mirrored by real life events after its release.
- In the 2030's, the world was hit by a series of pandemics, including an antibiotic-resistant strain of tuberculosis and a resurgence of the 1918 Spanish flu. These are said to have left the population noticeably smaller than it was a hundred years ago. For the record, the world population in 1930 was around 2 billion. And this is before the Gray Death hit.
- The sheer soulless evil of Bob Page. It's not enough for him to be a privileged billionaire living a life of luxury in a dystopian future in which everything is falling apart. All he cares about is becoming a "God", even if that means making everything ''even worse''.
- The fate of Lucius Debeers, left fully conscious but unable to move at all in some dingy basement. And even worse, his best "friend" has had the technology to cure him of this for years, but intentionally chooses not to and instead keeps Debeers in false hope.
- Many of the news bulletins are unsettling. It's mentioned in one NYC report that a recent chlorine spill into the Hudson has rendered drinking water poisonous to "an unknown extent". The bulletin then suggests that anyone made sick by the water heads to a free clinic. A free clinic likely packed out with people infected by the incurable and deadly Gray Death. So you can't even escape death by staying at home, due to supplies dwindling and no fresh water...your only other choice is taking your chances on a daily basis venturing into a crime-torn, disease ridden hellhole.