Kell dragons. They're the size of tanks, they're practically unkillable (especially in Jedi Knight, where bullets deflect off their scales), and they sport gruesome Slasher Smiles.
What's worse, in both encounters you'll have with them the developers specifically made sure it would be as nerve-wracking as possible. The first is in Jerec's palace, where you'll ride a long elevator down into a large room. As soon as the elevator touches down a door opens and the beast lumbers out at you. You can force jump up to safety on a narrow ledge just above it but you won't have enough explosives to kill it at this point. The only way out is to go inside the creature's holding pen and push a button on the wall that summons the elevator at an agonizingly slow rate. The second encounter is in a later level when you're exploring the Valley of the Jedi. After dropping down a shaft you fall into a dark pit. Stormtrooper body parts are scattered all around and before long you'll hear the sound of not one, but two Kell dragons. You'll need to either kill both of them or evade them long enough to find a way out of the dark pit in order to progress.
Gran and Trandoshans, who serve as Elite Mooks in Dark Forces before assuming a more prominent role in the early levels of Jedi Knight. Trandoshans can easily kill you from as much as an eighth of a mile away with their plasma-loaded "concussion rifles", while Gran toss grenades down on you from high platforms or windows.
Jedi Knight and Mysteries of the Sith have the Drugons, giant carnivorous fish which can kill you in one bite, can swim faster than you (if you're not using Force Speed), and may spot you from afar.
The Rancors in both Mysteries of the Sith and Jedi Academy can be this.
The one in Mystery of the Sith can not only kill you in two attacks, but it eats you alive when it defeats you, and you can only kill it with your lightsaber, which you won't have at first and must retrieve from a nearby vault after getting past the creature.
In Jedi Academy, on the Nar Kreeta mission, it's not as dangerous to deal with since you have ranged weapons that can affect it now, but it has a crapton of health, and it will just respawn if you kill it.
This can slide into Nightmare Retardant if you're playing around with console cheats. If you activate noclip mode right before the rancor eats you, it still bites of your model's torso, but then you can escape its grasp and run around as a pair of legs. Which still have full access to all your force powers. It's so over the top, it's more funny than Squick.
Dark Forces
In a moment of "what the hell, developers?", the corpses from the second mission of the first game are, upon inspection, sprite copies of the charred corpses of Uncle Owen and Aunt Beru, after the Imperial visit to the Lars home in Episode IV.
In the first Dark Forces, Kyle has just defeated the first Phase II Dark Trooper and is now escaping the facility. The only way out is through a dark, narrow hallway filled with the occasional Phase Is. The frantic music is pumping, the gas mask that was put on while placing the charges may still be on and in addition to its respirator adding to the atmosphere, you can hear the sound of metal feet on the metal ground throughout.
Walking through a dark sewer with very little light and no idea when a Dianoga will jump out. Seriously, they pop up out of the sewage and roar in your face.
Stage One Dark Troopers. They look like walking metal skeletons with swords and are partially bulletproof. And Lordy, those things are fast.
Jedi Knight: Dark Forces II
Gamorrean guards, at least after they Take a Level in Badass in Jedi Knight. Their evil piglike grunts are intimidating enough without those nasty battle-axes, which they can swing with deceptive speed. A couple of swings is enough to whittle your health down considerably, especially since melee attacks bypass your shields.
Boc is probably the most unnerving of all of Jerec's followers in Jedi Knight. Not only does he wield two lightsabers at once, but he jumps around so frantically (and so high) while fighting you that it's easy to get caught from behind. That, and he's definitely The Hyena of Jerec's crew, creepily giggling and cackling nonstop until you finally slaughter him - and even then, he continues laughing until he finally chokes on his last breath.
The scene following Maw's defeat in Jedi Knight. He dares Kyle to kill him. When Kyle refuses, Maw assumes he doesn't have it in him. He belittles Kyle as weak like his father and recounts how Jerec slowly killed him, saying it was “a death worthy of a coward” and how Maw put his head on a spike. He then lets out a downright chilling Evil Laugh before Kyle strikes him down.
That creepy-ass face in the 'Mysteries Of The Sith' loading screen that stares at you with those blank, empty eyes. This thing appears in every loading screen so you had better get used to it. Wondering what it is? Well good luck figuring it out, because you won't
It's possible it's one of those eerie statues on Drommund Kaas, but it's hard to tell for sure
The final three levels of 'MOTS'. Hoo boy
First, there's the swamps of Drommund Kaas. Right as you start you're in a big area with lots of shadows, creepy sound-effects, lightning flashing, and an ominous statue watching you. If you try to approach the statue, it's eyes glow white and a forcefield appears, stopping you from passing
Then there's the Sith Temple, the entire reason Kyle came here. It's important to note that this game implemented the use of coloured lighting a lot, so the result in some cases was that things were murky and dark on purpose, with thin shafts of light coming through from cracks in the walls and ceilings. And nowhere is this more apparent than in the Sith Temple
And THEN there's the Sith Temple Catacombs. God, it just doesn't end. Eerie sounds in the distance that sound like agonised shrieks, viciously powerful monsters lurking behind every corner, the fact that the very area itslf is crumbling and decaying, like some massive long-dead animal ready to collapse at any moment. You get this incredible feeling of utter isolation and death all around you, not just that you might die, but that the land itself is anticipating your demise
The Sith Zombies (exclusive to this final level) deserve a special mention, as does the area you encounter them. You're walking down a long, dimly lit corridor, and in the distance you can see what looks like one of unmoving the statues that you've seen so many times before, you're almost bored by them. Then suddenly, it spins around, pulling a full 180 to face you head-on. It's almost comical how fast it moves. Then the walls explode, and the Sith Zombies emerge
Skeletal corpses, groaning quietly, shambling along in the darkness, spamming Force Lightning on your ass. Think you'll enjoy killing them after that? Think again, because their death animation is their bodies falling apart piece by piece, while letting out one final gutteral, aching moan
Jedi Knight II: Jedi Outcast
Jedi Academy
Jedi Academy has the Sand Burrowers on Blenjeel, which can swallow you whole from underneath your feet if you're not careful and stand on the sand for too long. And that level is overpopulated with them, and you can hear them roaring constantly throughout the mission. They also have a very unsettling appearance, having multiple rows of teeth and no eyes. Even worse, some of the parts you need to collect to repair your ship are sitting right in the sand itself. They can be tricked into swallowing thermal detonators, but there are more Burrowers in the level than how many detonators you can carry, making them nothing more than a distraction for the Burrowers to go after. When the name for that level's save file is called "Danger", you know you're in for a nerve-racking experience.
