This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Black Hole
The exact nature of the crew of the Cygnus. Lobotomized by Reinhardt into mindless, subservient drones who can only continue to perform their tasks aboard the ship.
The ending of the film with regard to the crew. The last we see of Reinhardt and Maximillian is Reinhardt's eyes darting madly to and fro from within Maximilian's visor, undeniably insane, as the new entity rules over the wreckage of the Cygnus, and its mindless, but entirely innocent crew in what looks to be a massive fiery cavern.
The line graphs used to represent the titular space anomaly in the trailer, due to it looking so plain unnatural and being superimposed over the more terrifying moments of the film.
Just the brief glimpse of the "humanoid-robot's" face when Durant removes the reflective faceplate; although still clearly alive, the crewman's face almost looks like a corpse, and the eyes seem on the verge of hemorrhaging. That little raspy breath he lets out upon being unmasked doesn't help, either.
Maximillian's execution of Durant, which prompts Reinhardt himself to beg for protection from his creation. (Although he might just be playing the sympathy card — he curses out Maximilian for killing Alex, but Alex had just found out Reinhardt's secret.)
Dr. Reinhardt's pleas to Kate to protect him from Maximillian are another case of Nightmare Fuel should they be taken at face value, considering the possibility of spending decades practically all alone and having to look constantly over your shoulder to make sure your physically superior Dragon isn't trying to kill you.
What makes this all the more unsettling is that the movie was produced by Disney!