Nightmare Fuel / Xenonauts

• The fact that the Praetors haven't just nuked Earth back to the Stone Age to begin with. Turns out they're merely toying with humanity just to determine what they can use humans for. If they win, they change humanity into unrecognisable forms, just as they did the Caeseans and Sebilians before them. The Praetors modified the various races and took their notable traits for themselves.
• Reapers. The equivalent to X-COM's Chrysallids, they're fast and a single attack is not only a guaranteed One-Hit Kill, but also turns the victim into a zombie that becomes another Reaper upon death. Seeing a Reaper charge across the screen at ridiculous speed and zombify one of your troops for the first time is a harrowing experience and you'll be keeping watch for them from then on. As a corollary, the alien Dreadnought's cargo hold is filled with them, with the aliens' contingency plan being to drop them en masse into Earth's cities to run rampant. As it stands, during Operation Endgame, after six turns, six Reapers will spawn every turn, so you'd better blow those interlocks fairly quickly and have plenty of ammo and stun gas grenades if you want to get your men out alive. Oh, and they can turn up anywhere.
• The fact that the invasion is basically just a pet project of the High Praetor, who is only part of a much larger empire. An armada of more than three thousand craft is only a small part of the Praetor fleet. What happens when his superiors discover he's been defeated by a planet with a much lower technology level?
• Fridge Horror: You created a deterrent against the otherwise invincible all-conquering empire by making your impending conquest boring, inventing an FTL jammer that will force any invading empire to take centuries slogging their way through realspace to reach your insignificant blue planet and thus making Earth's conquest not worth it. Yay! Oh wait. You invented a solid means of creating a permanent barrier in the fabric of space-time, one which could embargo entire star systems for literal millennia, something so ludicrously ingenious that the Empire is flabbergasted. The more patient of Houses will slobber over the realization that you've invented the secret to crushing entire interstellar empires. Once their robot army finally manages to reach your descendants, they're going to enslave your species to re-invent the FTL jammers, cut off supply lines for the entire galaxy, and murder trillions through the collapse of interstellar civilization - if they don't figure out how to invent it themselves sooner. Great work.