What is the scariest Sci-Fi concept you've seen?

[Worldtree]

Anything like the leviathan is vast and incomprehensible but is almost too big to have feelings about, for me, in the way the death of a star is unstoppable and far away in the future.
And in OA , beings like The Thing could end up being your neighbors in the habitat in a nearby orbit, as long as they’re sophont and you have translation tools/Ontology platforms