Trent: Don't. Simon: (holding a shotgun below his chin) I have to, he wrote me this way.
Trent's first direct encounter with Cane. Cane reveals to him that he's nothing more than a character in the titular upcoming novel, and that he has no choice but to deliver the manuscript of said novel to Harglow. Then he rips himself like paper, revealing a black void from which an army of horrific entities — likely the Old Ones themselves — appear and chase Trent down the tunnel that Cane had just made.
Trent's final monologue to Dr. Wren is, by itself, probably one of the grimmest things about the movie.
Trent: Every species can smell its own extinction. The last ones left won't have a pretty time with it. In ten years — maybe less — the human race will just be a bedtime story for the children. A myth, nothing more.
The ending. Just ... the ending. Monsters break into the asylum and kill everyone, Trent escapes, and overhears via an emergency broadcast that the world is rapidly succumbing to an epidemic of insanity and mutation into horrible creatures, all induced by the release of the titular book and its film adaptation. The very filmyou're watching.