This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Knights of the Old Republic
Rakghouls: The zombies in the Star Wars universe
In the first game, the first time you see Malak without his jaw prosthesis.
Korriban In the first game, it's a small world with a Sith academy. The player is required to explore the tombs of dead ancient Sith, which even the powerful Sith at the academy are afraid to enter. In one, you have to have a conversation with the Force-ghost of a long-dead Sith Lord. In another, you're captured by a completely insane psycho who asks you a series of questions, and if you get them wrong he'll torture you with lightning. If you get them right? He'll torture someone else instead. This is added to by the fact that the Sith at the academy are casually cruel, to the point that they'll torture applicants to the academy until they collapse, leave them for dead, and then not even let them into the academy. Nearby, you can explore a cave filled with giant flying reptile things which are more annoying than scary, but also in the cave is a terentatek (think a rancor, but covered in spikes and resistant to Force powers). In one of the tombs, there are two more which you have to fight at the same time. And one of them can kill your entire party if you're not very careful.
If you sit down and think of it for a while, Bastila turning to the Dark Side falls under this: it is stated that in order for that to happen, she got tortured and had her mind broken... ouch.
The Hrakert Rift station on Manaan in the first game. Full of insane Selkath that try to rip you to pieces inside, surrounded by firaxen sharks that try to rip you apart outside. The outside is actually worse because you can't see them until they're practically right on top of you and your targeting system picks them up. And there are more beyond the edges of the map. Not to mention that you can see the massive one hovering by the bridge, ready to eat you if you try to cross over to the Star Map before you've done something to get rid of it, either by destroying the kolto filters or poisoning the water.
Each team member's insight of the unknown planet is chilling in its own right, but the one that stand out the most is Canderous. He has up to this point been the groups stoic Mr. Seen It All and he's finally found a planet that's alien even to him, making him deliver this chilly description.
Canderous: This world is... odd. It looks like a battlefield, but the environment is lush and green. Whatever happened here was a long time ago. I've seen many strange worlds in my time and this is the strangest still. (nervously) I don't like this place. Something happened, or will happen, and I don't like it. We should get out of here as soon as possible.
One of the most horrificdark side actions you can do in the game is using force persuasion on Zaalbar to murder his longtime companion Mission Vao.You Monster!
The rakatan computer guarding the Star Map in the Shadowlands on Kashyyyk. All you know about it is that it predates the Republic by 30,000 years and that it approves of dark-side answers. The music doesn't help, and what Jolee has to say about it is somewhat chilling too.
Jolee: Well, well, a Star Map. An ancient artifact of dark side power. Can't say I'm surprised. I always knew there was something funny down here. I wonder if the Star Map has had an effect on the evolution of the creatures here in the Shadowlands. Might explain why it's so dangerous down here.
The Star Maps in general can be pretty creepy, especially if you look into the lore not extrapolated upon in the game. Semi-sentient artifacts infused with the dark side that are nearly indestructible - and even when damaged, they are capable of self-repair. The one on Kashyyyk was thought to turn what was once probably a very Earth-like planet into what it then was, and the one on Manaan is likely what spurred on the spontaneous evolution of Firaxa and Selkath.
Even worse is that the prisoner says that you aren't the first person to enter his prison. The others panicked and ran off into the white void that surrounds you, and according to the prisoner, are still out there.
And the real nail in the coffin? You're the only one able to understand the Rakatan, because as Revan, you used the Force to rip their language from their minds, and despite losing most of your memory, the fluency of that language thankfully remained. They are otherwise a completely forgotten race with a language that likely no other person in the galaxy except maybe Malak. Maybe. would know.
During one of your conversations with Canderous, he'll talk about an encounter a Mandalorian raiding fleet he was part of had with a ship that they thought was an asteroid. His description of how it looked and acted perfectly matches a Yorik-stronha, a Yuuzhan Vong spy ship. Thing is, this is four thousand years before the Vong came to the Galaxy, so how the fuck did this thing get here?!