This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Incarceron
Incarceron is running out of organic resources, so sometimes people and animals are 'born' with mechanical parts. And organic matter is recycled.
Finn's memories of his first few years there.
Though rare, some people are literally born via Incarceron.
Finn encountering the Beast in the Cave.
The Half-Men are people who have been "repaired" by Incarceron with robotic components.
The Chain-gangs are groups of people fused together and sharing a consciousness. They assimilate their victims.
Incarceron and its eyes are everywhere, it can speak to you at any time, and it's very proficient at breaking people.
If Incarceron were to Escape, the entire prison would suffocate, freeze, and die in darkness.
The Prison's threat to Rix in Sapphique.
"You will tell me where the Glove is and then for your reward I will very slowly and very carefully destroy you, atom by atom, for centuries. You will scream like the prisoners in your patchbooks, like Prometheus eaten daily by the eagle, like Loki as poison drips on his face. When I have Escaped and everyone else is dead your struggles will still convulse the Prison."