YMMV / Tomb of Annihilation

• Nightmare Fuel: The Atropal, dear god. It can be best described as an undead god-fetus.
• That One Boss: Shifts between Anticlimax Boss and That One Boss, but the Atropal qualifies as one or the other, depending on how you attack it. On the one hand, it has vulnerability to radiant damage, low hitpoints, and low dexterity, making it easy to kill quickly. If you aren't able to take advantage of that, though, its Wail attack can give every player a level of exhaustion. If it gets multiple wails off before dying, the party will be at a hopeless disadvantage against Acererak, even with the 11th-Hour Superpower.
• The Woobie: Nepartak, the ten-year-old girl Acererak killed to make a flameskull for his tomb. It's not even that tough of a fight, since the players will most likely be around lv. 9,^note  Flameskulls have a challenge rating of 5, and the undead spiders that accompany her have a CR of ½ each. but her screams will never fail to remind the players that they're fighting a scared little girl.