Personally, I find any fate involving infinite or near-infinite time dilation/looping to be the most unsettling. The concept of eternity is already so far beyond our human ability to functionally grasp and a bit disconcerting even in the best of situations. If such an eternity involves purposeful torment, the result is just horrifying to me. In other words, I’m most afraid of those situations that make death by even the most excruciating mechanism seem like a mercy.
SPOILERS AHEAD for “The Endless” (film) and “The Long Dream” (short story)
For instance, in the film “The Endless,” the scene in which a character is forced to relive a gruesome death for eternity in a singular, seconds-long loop — all while maintaining an ongoing line of consciousness — has always given me the chills. After countless repetitions, the looping death would almost seem like an afterthought in comparison to the hopelessly inescapable nature of your plight.
If we count Junji Ito’s short story/manga “The Long Dream” as cosmic horror, the concept of an increasingly long series of phantasmagoric nightmares (making it such that you spend millions or even billions of years trapped in a hellish, alien dreamscape each time you sleep, while everyone else experiences time normally) also freaks me out.