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[[folder:The Lost Hero]]
* Enceladus kidnaps Piper's father and threatens to devour him…unless Piper does what he tells her to. That includes not telling anyone. Eventually the giant gives her some directions: [[SadisticChoice derail the quest and have Jason and Leo killed, one way or another; or her father dies]].
%%* Khione goes beyond merely unpleasant after TheReveal.
* Leo killing his mother. By accident, of course, after Gaea's emergence, but try telling ''that'' to the cops and his mother's family.
* The encounter with the Cyclopses in Detroit gets on here by echoing Annabeth's earlier encounter mentioned in ''The Sea of Monsters''. First, the three demigods are forced to separate. Piper's still reeling from a twisted ankle and hears Jason's voice…coming from a looming one-eyed figure. Later, Leo rushes back to the building and hears Piper's voice…but stops, because he realizes that the person speaking ''doesn't smell like Piper''. The Cyclops impersonating Piper drops it mid-sentence and heads back inside to start cooking Piper and Jason, both captured and dangling from the ceiling. If not for Leo's decision to say and fight, his friends would have died.
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[[folder:The Son of Neptune]]
* The description of the last moments of Hazel's first life, as oil fills her lungs and the chamber collapses.
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[[folder:The Mark of Athena]]
* The things Nico went through in Tartarus are implied to be [[NothingIsScarier too horrible to say.]]
* The ''description'' of Nico when he's freed. Half-dead, ''horribly'' thin, using his sword for support, and likely traumatized.
* Annabeth stepping on and surrounded by the skeletons of all her siblings who died failing to complete the Mark of Athena's quest.

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[[folder:The House of Hades]]
* Tartarus. It's not just a place, but literally the body of the god Tartarus. Nightmare Fuel with a good dose of NauseaFuel.
** That is not the worst of it, as Tartarus is not a God but one of the "first-born" from Chaos. He is not a god; he is literally the personification of what they represent, while the land of Tartarus is his real body.
** There's also the descriptions of Tartarus, such as Percy and Annabeth breathing poison, drinking fire, and the little fact that the ground appears to be made of '''skin'''.
* The arai are some of the creepiest monsters in the series. They look like harpies, with the bodies of birds and the faces of old wrinkled women, but with the addition of some ''serious'' UncannyValley in that they have even ''more'' monstrous features: their faces don't move when they talk, and the voices that communicate for them all seem to come from some sort of hive like chorus completely independent of the creatures themselves. Yet even that isn't the creepiest part, as they are also the literal embodiment of curses. Meaning if you destroy one you get inflicted with a curse by someone you once wronged or did harm to, hoping that you one day feel the same pain as they did. This, horrifyingly enough, also includes monsters, and Percy and Annabeth have killed ''a lot'' of monsters. In their cases, Annabeth goes blind from Polyphemus cursing her after their encounter in ''The Sea of Monsters'', and Percy experiences the wound that killed Geryon when he faced him in ''The Battle of The Labyrinth''. The worst one however would have to be where Annabeth kills an ara and receives a curse made by ''Calypso'' to never be with Percy again, and she's left stumbling around blind and alone as she believes he's left her. This has Percy dawn on the horrible realization of how bitter Calypso must have been after he left her, and how even though he made the gods vow to free her, he never followed up on their promise.
* Tartarus is so terrifying that Percy literally drops his sword. Heck, he took out two Titans while creating a semi-physical form (which is not his real form; it's a FightingAShadow-type form)! Now imagine what happens when Gaia (aka Earth) wakes up... "Pray she never fully awake" indeed.
* Cupid. He wastes no time proving that Love Hurts, and that he revels in it. First, he [[CurbStompBattle curb stomps]] Jason, the latter only scoring a glancing blow on Cupid's intangible form. The abuse only stops when Nico submits to Cupid's "test". The god promptly [[BreaktheCutie psychologically breaks him]], apparently for the hell of it, before giving them what they seek.
-->'''Cupid:''' [They] have met my counterpart, Thanatos. We are not so different. Except Death is sometimes kinder.
** For many LGBT+ readers, this scene was the most terrifying part of the book. Coming out of the closet is something that takes months, or even ''years'' to prepare for, and is still an anxiety-ridden process. Yet it happens to [[ArmouredClosetGay Nico]] without him having a say in the matter, when he couldn't even admit it to himself. For all he knew, Jason could have been a homophobe, or could have [[TeensAreMonsters ratted him out to the rest of the team.]] And considering [[UsefulNotes/FascistItaly when and where]] Nico is from, he was raised knowing that could lead to isolation, imprisonment, [[CureYourGays or]] [[UsefulNotes/TheHolocaust worse]]. He is the most genuinely afraid we've seen him-- and because of something that could happen to anyone, anytime.
* While choking on poison, Percy remembers that poison is still liquid... something he can control. Cue to him nearly choking Misery with her own fumes, while his form still had the appearance of a decrepit zombie-ghost-thing. On top of that, he reverts to his traditional method of fighting at the brink of death-turning his terror into anger that fuels him for combat. He promptly churns every ounce of misery and turmoil she was heaping upon them back towards her in a dark rage. It's no wonder even Annabeth was terrified of him at that moment.
** Even worse, we see it through Percy's eyes, get to see every ugly inch of his attacks. ''[[EvilFeelsGood He likes it.]]'' Just one book ago, Piper was able to tell something was wrong when Percy was possessed by the cruel look on his face. Now, he's savoring Akhlys' pain like it's vermouth. And when Annabeth begs him not to touch the power again, he seems dazed and reluctant to promise her....
* Nyx's appearance is ''mostly'' light-hearted and humorous, but given that literally every being in Tartarus fears her, as does ''Zeus himself'', the dread that's been built up around her means that even with the goofy way Percy and Annabeth thwart her she still feels ''very'' menacing. After they escape her, Koios tells Iapetus that Polybotes believed the two of them had been "swallowed by Nyx", which, given her immense size and Akhlys's mention of "feeding [Percy and Annabeth] to Mother Night", [[SwallowedWhole might be very literal]].
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[[folder:The Blood Of Olympus]]
* Nico outright kills a guy by turning him into a ghost and banishing him to the underworld. Needless to say, Reyna and Coach Hedge were thoroughly unnerved by this.
** Worse, Nico doesn't just kill him. He erases his identity, his memories, his personality and banishes him to The Fields of Asphodel forever. And he delivers a PreMortemOneLiner that could rival Sansa Stark:
-->'''Nico:''' [[YouAreAlreadyDead You're already dead.]] You're a ghost with no tongue, no memory. You won't be sharing any secrets.\\
'''Bryce:''' No! No, I am Bryce Lawrence! I'm alive!\\
'''Nico:''' [[UnPerson Who are you?]]
* Nico stops Will Solace from telling Octavian that he's snagged in the catapult. Mike Kahale then defends Octavian and tells him to fire the shot. After he's launched skyward to his death, it's revealed Kahale knew what would happen. In-universe example as well, as Nico spends the next half a chapter worrying about Will's reaction to what he saw.
-->'''Kahale:''' Goodbye, Octavian.
** The guy's SanitySlippage is also unpleasant. What's worse than regular Octavian? An unhinged Octavian, acting as a Roman commander.
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