Thief : The Metal Age mostly lacks supernatural or horror elements, but has plenty of nightmare fuel afoot, just like the other two games. The Masked Servants of the Mechanists, for example. And Karras's recorded message to Garrett will make you SHUDDER:
"I would have had thee under my command, else dead! Indeed... I'd have had both..."
In "Shipping... and Receiving", you can gain access to the Mynell Steaks building to steal the Mynell's recipe, only to find out that the secret ingredient was spider meat, and some of the spiders in the room are still alive, instantly trying to attack you. They hardly cause any harm, but considering that it's the first possible monster encounter in the game, placed in an otherwise ordinary mission, inexperienced player (and especially the one who plays any Thief game for the first time) can be quickly caught off guard.
In the same mission, the player can explore a small pirate vessel at the backside of Building B. Exploring it reveals a corpse lying in one of their shipping containers.
Although he was without a doubt harmless, most players will agree that the "Golden child" (Mechanist Cherub) that appears out of thin air after reading the scripture and turning back into a small room that you just went through to get to the scripture gave them a huge jump and unnerved them. What's even worse, it makes highly odd noises, is invincible, tries to attract the guards and acts very oddly. Then you're wondering for the rest of the level whether it is just a robot or if there is something living inside.
Karras is an incredibly effective source of nightmare fuel, maybe because of his amusing speech impediment making him seem quite laughable early in the game. But as you play on, you come across documents, and overhear conversations, that slowly reveal just how broken Karras is. He has the power of a dominant church, ambition unrestrained by empathy, and he hates everyone. He doesn't even care for his followers.
Then there's Karras' master plan. The Servants he's been handing out to the nobility are primed to release a rust-like substance known as the Necrotic Mutox, which breaks down everything remotely organic in a violent reaction that produces even more Necrotic Mutox (at least until it settles and dissipates). Seeing as noble households generally have large gardens, they'd be the focal points for a city-wide chain reaction that would wipe out all organic life in The City, and possibly beyond. All so that Karras can replace it with his twisted vision of the Builder's Paradise, filled with machines built in his image in the place of people.
The fact that he correctly guessed you'd crash his party at the tower on his own hunch, refused to attend despite all his followers around and set up a trap is a disturbing indication of his huge intellect and paranoia.
The Thief games are filled with moments specifically designed to make you jump out of your skin. The worst (excluding Shalebridge, obviously) is probably, as mentioned previously, that goddamn mechanical baby in the Mechanist tower.
The Mechanist robots in Thief II are very intimidating to the inexperienced player. Also, the invincible but harmless Mechanist 'Cherub' that appears out of thin air and follows you around everywhere in one level making weird noises seriously scared a lot of players who weren't unhinged by the scarier elements of the first game.
The Mechanist servants, when you realize that they are free willed humans with their actions controlled by robotic implants. They often have conversations with themselves in which the human bemoans their fate or the robot tells the human off. A specific few are known to utter 'thank you' with their final breath if you kill them.
The first place you encounter them is a laboratory that's part of a converted Hammerite seminary. Here are tanks in which are found servants that are apparently partway through the transformation process. Charming. Underneath the floor, however, is a collection of dead bodies... "subjects" that didn't survive whatever unspeakable "processing" they undergo? Like Garrett says, "I could really learn to hate these guys."
The jail cells of Shoalsgate. Here are people living in dark and dank (and presumably smelly) cells, some implied to be unjustly incarcerated for the smallest of crimes. What's more, it's always patrolled by a Hammer Haunt that doesn't even acknowledge your presence. But worst of all is the seclusion cell, wherein sits an insane murderer, who continually "reminisces" about his crimes (only stopping every once in a while for fits of insane laughter). And worst of all? You never see what the guy looks like. The point is... if you're a criminal in the world of Thief, NEVER GET CAUGHT.
During the "Eavesdropping" mission, Garrett intercepts a meeting between Sheriff Truart & Father Karras in which the horrible power of the mechanical servants is revealed. From behind a door you can hear the sound of rust gas pouring out of the servant and horrifically killing a beggar that was used as a guinea pig.
Before the beggar is brought in to demonstrate, Father Karras makes reference to a "most unfortunate guttersnipe who waits just outside." Since you're listening in from behind a door it's jump-worthy, because for a brief second you assume he's referring to you.
In "Blackmail", there is a hidden basement containing a rack and (depending on the version you're playing, a dead or decidedly un-dead) corpse. Just what the hell was Truart up to?
In “Life of the Party”, Garrett can stumble into an elevator leading up to a room with a decidedly creepy-looking altar, containing a book... which, if he decides to read, turns out to be the Book of Ash mentioned with fear by the mages in the first game, complete with a full ritual in Lovecraftesque... and upon closing it, the player will find its previous two readers, turned into undead monsters!
In "Trail of Blood", in the cavern area, there is a room in which, as soon as you walk into, you will find a plethora of eyeball-flowers, (Pictured Above) which will all too quickly start turning to stare directly at Garrett, with squick-inducing squelching noises.
The mission also features a Pagan village strewn with the corpses and restless spirits of its former inhabitants, who had been recently massacred by the Mechanists. One of them is a young girl, who seemingly died clutching her doll. As Garrett approaches her body, the girl's spirit appears and gives Garrett permission to take her doll with him, saying that she doesn't need it anymore.
The end of the mission is in a secluded forested area letting out all kinds of unsettling noises, giving off the impression that there's something waiting just out of your vision, ready to strike. If you are careless in where you step, you will find some of the trees can move. And then you finish the mission and Viktoria shows up. Her entire conversation with Garrett gives the impression that she could effortlessly kill him at any time, and has perfect reason to due to him killing the Trickster, and only holds back because she needs him.
"Sabotage at Soulforge" lives up to its status as the final level. Being the heart of the Mechanist religion, it's a sprawling factory-church brimming with hostile robots and hazardous machinery. Then there's the entry hall, filled with the remains of Viktoria's sacrifice, which serves as a fuel source for the Necrotic Mutox spread by the recalled Masked Servants, killing Karras at the hands of his own weapon.
While most of the enemies on the level are robots, there's also a few Masked Servants seemingly pushed into combat duty. Looking closer at their uniforms reveals that they're covered in blood, as though they were worn while the Servant conversion process was undertaken. It's implied that Karras has begun subjecting his own followers to the Servant process, essentially killing them and making organic robots of their corpses.
While he absolutely had it coming, Karras' end isn't exactly pretty. After sabotaging the relay beacons so the Servants expel the Pox within the Cathedral, Garret seals the Cathedral as he leaves so Karras can't escape. As the plant life created from Viktoria's sacrifice causes the released Pox to go into overdrive, Karras is seen giving a sermon to his "children", preparing for his ultimate victory, only to trail off when he notices a Servant suddenly standing in the hall, expelling the Pox within it. Karras has just enough time to realize that something is horribly wrong before a tidal wave of Pox gas surges forward and consumes him.
The final cutscene has a brief shot of a stricken Mechanist combat robot lumbering down a corridor until it powers down. Before it does, it says one of the prerecorded phrases that Karras made for his mechanical "children". However, given the first-person language and the situation during the ending, it could almost pass for the robot having gained sentience and reflecting on its life before it dies.
"When I was very young, Karras took me away from the foundry mother and said, 'thou art the child of my endeavours. Follow me and thou shalt... inherit the Earth...'"
