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Nightmare Fuel: Star Trek Online
During the Mission "What Lies Beneath" the player is trapped in a small, poorly maintained part of Drozana Station with practically no lighting. Then, out of nowhere, you hear... this
During the Tie-In novel, The Needs of the Many we get treated to understanding what it is like to be an Undine infiltrator. Unlike the Changelings of Star Trek: Deep Space Nine, the Undine don't just copy your body, they copy every part of your body including your internal organs and how your neurons fire in your brain. This means they don't just copy you physically, they copy your soul. Worf in particular had to deal with this threat after he was injured in a battle and Undine tried to replace him, but due to his injuries he had a memory lapse during the struggle and the next thing he remembered was seeing himself be thrown into a Vulcan body disintegrator (a means of disposing dead bodies since the Katra is what matters, not the body). For the next 30 years Worf is horrifying confused as to whether he's Worf, Son of Mogh or an Undine who believes he's Worf and that the Worf personality overwrote his Undine one so thoroughly that he's just complete fake. Jake Sisko notes that he can blatantly see TERROR in Worf's body language. The worst part is, for a Genre Savvy reader, who's full aware of the trope that Worf made, you know it's highly possible that this is an Undine who thinks its Worf. The Paranoia Fuel of this concept literally is what made the game and the entirety of the STO universe bloody terrifying.
Slightly downplayed in one the game's missions: the Undine still need regular injections to maintain their assumed form, so there is a relatively easy way to test it if you think you might be an Undine, assuming you have time and reliable allies that you think aren't Undine.
In Colosseum, a mission in the Cloaked Intentions story arc, you return to your ship from a fairly easy away mission and wake up in a prison run by brainwashed Tal Shiar working for the Iconians. You're left to wonder how you ended up in this predicament when during your escape from the arena, you see it. An Iconian gate. And you realize how you ended up in their arena, they just walked into your quarters or your ready room and took you. The Iconians can literally just kidnap you without anyone ever knowing and take you somewhere no one will ever find you. Pure Paranoia Fuel.
It's more implied they intercepted your transporter beam, but the paranoia remains.
The bad future in the timeline created by the Enterprise-C's jumps to the TNG and STO era. First, the Feds lost the war but continued to fight guerilla warfare. However, at the same time, a Bajor that was never conquered by the Cardassians discovered the Wormhole and the Dominion swarmed in. With no Starfleet, Section 31 or The Sisko to stop them, they cubstomped the KDF and Romulans and destroyed Earth. The Tholians were spared by signing a non-agression treaty (as seen in DS 9) and control Tau Dewa where the player and their starship is now a freighter serving the Tholians. Along the way you meet your 3 most senior Bridge Officers who are run down people who never experience joy in their lives, and meet many of your friends from other STO missions. It says something when Admiral T'Nae, a Vulcan, doesnt even bother being coldly logical anymore and blatantly emotes depression. And to add some Fridge Horror to the mix, because the Romulans were defeated, the Iconians never destroyed Romulus, which means the JJ Timeline ceased to exist. Yes, that's right. The Enterprise-C's time travel adventure nearly caused TWO timelines to face non-existence.
With Legacy of Romulus, we get a new look at the Elachi...and it isn't pretty. It turns out that they are trying to convert Romulan colonists into creatures like themselves. The results are anything but pretty and anyone you cannot save you decide you need to destroy to be merciful.
For bonus points with Fridge Horror. all those Elachi you killed? They could have been your friends and family from your colony. Though it was probably a Mercy Kill.