This is based on opinion. Please don't list it on a work's trope example list.
Nightmarefuel / Transformers: Fall of Cybertron
The Insecticon hives. They're, of course, swarming with Insecticons that come from every direction to attack Autobots and Decepticons, and the whole organic look just adds to the fear factor. Playing as Grimlock, you'll see these things come out of the ground and drag Decepticons down underneath it. To top it all off, you sometimes hear whispering-like voices coming from that sound like ghosts, which basically come down to seeing you as food.
Grimlock states that he was aware of what Shockwave did to him, implying he was conscious while he essentially had his body torn apart and rebuilt.
When Grimlock and Swoop go through Shockwave's lab, you'll see a few Insecticons that didn't survive the experiments. Fiddling with buttons can cause things like a saw cutting into one of them.
Sharpshot's death. Not that he didn't deserve it.
The Sea of Rust. Perceptor's words say it all:
Perceptor: Never have I observed such a traumatizing sight. An entire city of rust. Everything down to the last bolt is encrusted. Severe oxidation, pitting the metal's surface, gradually deforming its shape. I fear for the health of all Autobots. The supply of Energon may be on every Cybertronian's mind. But I have observed the encroaching and effects of rust on many Autobots. We must be ever vigilant to its presence, or this city will overtake us all.
The Riot Cannon for your enemies. Anyone hit with one single shot will be BLOWN APART. Arms, legs, torso and head go absolutely everywhere. It just shows how terrifyingly brutal Cybertronian weaponry can be, particularly when compared to ours.