The description of Nico when he's freed. Half dead, horribly thin, using his sword for support, and likely traumatized.
Arachne, full stop.
The House of Hades
Tartarus. All of it. Especially since it's not just a place, but literally the body of the god Tartarus. Nightmare Fuel with a good dose of Nausea Fuel.
That is not the worst of it, as Tartarus is not a God but a one of the "first-born" from Chaos, he is not a God but literally the personification of what they represent, while the land of Tartarus is his real body.
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 Fighting a Shadow-type form)!!! Now imagine what happens when GAIA (a.k.a Earth) wakes up... "Pray she never fully awake" indeed.
Cupid. Mother-Fucking-H. Christ-on-a-cracker Cupid. He wastes no time proving that Love Hurts, and that he revels in. First, he 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 psychologically breaks him, apparently for the hell of it, beforegiving them what they seek.
Cupid: [They] have met my counterpart, Thanatos. We are not so different. Except Death is sometimes kinder.
While choking on poison, Percy remembers that poison is still liquid... aka 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. It's no wonder even Annabeth was terrified of him at that moment.