This game may have been thought to be LighterAndSofter by the graphics but [[NightmareFuel oh boy]], ''[[BloodierAndGorier they were wrong!]]''
* In general, hearing the deathcry of your character. Special mention goes to the female wizard.
* [[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.
** To make things worse, in one of the later entries of this journal, Leoric states that [[AndIMustScream he is aware of his madness, but powerless to stop his horrific actions.]]
* 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.
** [[OurAngelsAreDifferent Malthael]] himself has become this. Originally he was The Archangel of Wisdom and the Leader of the Angiris Council but went mad sometime and now he has become the Angel of Death. [[ I dare thee mortal]].
** Cain's description of the Butcher is disturbing for a completely different reason: [[spoiler:He describes the horrifying demonic Butcher'''''s'''''. That's right, there are ''more'' of them.]]
* The ''[[ Reaper of Souls]]'' intro has this with Malthael, specifically when, while he's butchering the Horadrim, we get to see his powers up close when he literally ''causes a Horadrim to wither and decay, ripping his screaming, struggling soul out of his body.'' Tyrael puts it best:
-->...You should all... ''[[OhCrap run.]]''
* The first chunk of Act V, especially before you get to your first settlement. First you see a couple people running and screaming...par for the course, honestly, by now. Then you see a large group of people running, only to see their souls yanked from their bodies, turning them into Undead enemies to fight you. You progress, and are told about [[HopeSpot survivors in the church]]. You reach the church, only to see a jet of darkness burst from the doors. You hear pained screams for an instant, before they are rendered silent. Even as you play through Westmarch, you continue to see some...[[ParanoiaFuel not all, but some]] of the soldiers get their souls pulled, turning them into Revenants, too.
* When you get close to the Soul Crucibles that you need to destroy, there are dead bodies everywhere, just piled up in the streets. And you have to walk over them.
* The [[spoiler:Grand Maester]]'s plan during ''The Templar Reckoning'' quest for Kormac. [[spoiler:This utter bastard plans to implement the same horrendous [[ColdBloodedTorture initiation ritual]] that resulted in [[BrainwashedAndCrazy what happened to Kormac and Jondar]] and apply it on a grander scale, doing this to ''every citizen of Westmarch and beyond'' in order to make them Templars]]. It's little wonder that Kormac decides that [[spoiler:the Maester, and the Order itself, need to end]].
* Malthael's plan during Reaper of Souls. Tearing one Horadrim's soul out in front of Tyrael and eating it is disturbing. Seeing [[spoiler:the Soul Crucibles in Westmarch]], and the floor littered with bodies from the Reaper attack is horrifying. Then you learn from Tyrael and his fragment of the stolen Black Soulstone that [[spoiler:Malthael is planning to use the soulstone to forcibly contain all demonic essence in the plane the Soulstone is in - and he's about to send it into Sanctuary. And [[KillEmAll human souls are half-demonic.]]]] By the time you reach him, he has a LOT of souls under his control, plus [[SealedEvilInACan the Black Soulstone and its contents]]... And Malthael [[OneWingedAngel eats souls.]]
* [[spoiler: The implications of Malthael's successful release of the Black Soulstone on Sanctuary. He tooled it to absorb all demons on the plane. And humans are all half demon. No one gives a clear answer to what this means, but everyone agrees that there is no way that it won't result in the death of every human on the plane.]]
* The following conversation with Eirena comes out of nowhere and isn't expanded upon either. The location at which she says it, such as King Leoric's mansion with its gruesome backstory, doesn't make things better either.
-->'''Eirena:''' There are two boys playing over there. Can you see them?\\
'''Player:''' What do you mean? There is nothing there.\\
'''Eirena:''' ''*cheerily*'' Never mind. They must be dead.