Big-Lipped Alligator Moment: The Danse Macabre is an in-universe example. One day every 80 years or so (which is probably not coincidentally about once in a human lifetime), white flowers bloom in the graveyard and The Grim Reaper leads a dance between the living and the dead. Even people who are not aware that ghosts exist proceed about like this is normal, but the living cannot remember it after it happens and the dead are forbidden from talking about it with the living. Those who are not living or dead, such as Silas, cannot participate in the Danse Macabre and Bod questions him about the sheer BLAM-ness of it after he finds out that none of the ghosts will discuss the events of the previous night.
Complete Monster: "The man" Jack Frost is the most dangerous and wicked assassin of the Jacks of All Trades, and is the main antagonist of the novel. First seen after murdering an innocent child and her parents, Frost tries to stab a baby to death in order to prevent him from growing up to become a threat to his organization, and, when the toddler escapes, Frost nearly murders an innocent man while in his pursuit of the child. Years later, Frost returns to kill the boy, now a teen named Nobody "Bod" Owens, and, after sending his fellow Jacks after the boy, knowing they will most likely fail and die, Frost kidnaps Bod's best friend, Scarlett, and tries to kill both her and Bod for "pride in his work", at which point Frost plans to use the Sleer to remake his organization in his image and make himself all-powerful. Sociopathic and cruel, Jack Frost is a dark villain to match this dark novel.
Esoteric Happy Ending: Bod leaves the graveyard at 15, with a passport, a small suitcase, and a wallet which explicitly carries a good, but not huge amount of money. He's a Fish out of Water with ethics and mindset of the dead, a large fraction of his education was in using powers he no longer has, and he has barely any experience with normal human interactions (he attended school for barely a few months and went to a restaurant for the first time less than a year ago). He wants to travel the world and experience life, but has neither the means nor skills to do so. Between being a minor too young to even get a minimum wage job, homeless, and without a diploma or marketable skill to his name, Bod is not up for a happy next few years.
To be more specific, the part where Jack Frost is "protected" by the Sleer. Although there are a few more segments that could fit this trope, like the fate of Nick and Maureen. But none are as bad as that first one.