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 these poor souls does not help in the slightest. 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.
That's only Nightmare Fuel if you assume the figures seen in the hellscape are the actual crew members' souls, and not just another element of the environment custom-made to torment Reinhardt himself. It's possible that their actual spirits departed their bodies when they underwent the brain surgery that turned them into drones, long before the Cygnus entered the black hole.
Then there is his 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.)
What makes this all the more unsettling is that the movie was produced by Disney!