This game may have been thought to be LighterAndSofter by the graphics but...''[[NightmareFuel they were wrong!]]''
* [[spoiler:Adria's betrayal]], leading to [[spoiler:Diablo possessing Leah in one of the most horrific deaths in the entire series]] and [[spoiler:the rebirth of Diablo, the Lord of Terror himself, as the embodiment of all seven Great Evils in one being, the Prime Evil. And worse, Diablo wants to [[HellInvadesHeaven destroy the High Heavens themselves]]]]. [[HolyShitQuotient Holy Shit]] does not even begin to describe it.
** [[spoiler:The invasion of Heaven itself. The High Heavens are supposed to be a place full of holy and awesomeness, even if a lot of the angels are dicks. But when Diablo invades, you're treated to sights like angels being tortured, ugly demonic cysts that you have to destroy to continue and the place becoming more and more Hellish as you progress to the Crystal Arch. And as mentioned below, not even angels are immune to Diablo's evil!]]
* Take a moment to listen to the music of [[spoiler: Whimsyshire]] on headphones, full volume, in a dark room, and without looking at the game. [[note]]As you might have guessed, the music is actually a real musical piece being played backwards. The original piece is called ''Gaspard de la Nuit - Le Gibet'' by Ravel. [[/note]]
* The Halls of Agony in general. These were King Leoric's old torture chambers, which saw plenty of use back when he was still alive and going insane due to Diablo's attempts to possess him and the evil whisperings of his EvilChancellor Lazarus, claiming the lives of quite a lot of people, up to and including his own queen. And that was before Maghda and her coven decided to appropriate the place for their twisted rituals. What makes things even worse is reading his journal, which captures his descent from noble king to stark raving mad step by step. The Darkening was not a fun time to live through.
* The massacre of Alcarnus in Act II, though not shown onscreen, showcases quite vividly just how evil Maghda and her Coven are. These are people whose primary means of fueling their magic (even illusions) involves ColdBloodedTorture, and Maghda at this point is [[VillainousBreakdown getting desperate]].
* Ghom is a rare case of turning the BigEater trope into NightmareFuel and NauseaFuel. While you are fighting him, you can get your hands on his journal, which narrates how he forced his human prisoners into ''[[IAmAHumanitarian eating their own comrades]]'', only to eat them himself after that.
* Listen very carefully during the battle with Diablo. You can hear screaming, just high enough to hear, but not louder than the background noise. It's surprisingly creepy.
** It's even worse because [[spoiler:you know who is supplying the screaming. Poor Leah...]]
* The descriptions of some of the monsters are pure nightmare as well.
** One would expect the Tormented Stingers, for example, to be scorpion-like demons out of hell. But instead, these monsters are created by twisting the bodies of human sacrifices into these horrific forms.
** The Unburied are a hideous amalgamation of corpses flung into mass graves, animated by foul magic.
** The Morlu, who you fight near the end of the game in Act IV, are the ultimate fate of mortal sorcerers who make deals with demons.
** [[spoiler:Corrupted Angels]] in general, probably the perfect embodiment of [[FromBadToWorse just how bad things have gotten]] in Act IV.