This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Planescape: Torment
This trailer makes the game look like a horror fantasy, complete with creepy music and imagery.
The Pillar of Skulls. It's a massive tower made up of the skulls of Manipulative BastardConsummate Liars who are aware and screaming in agony. If you have Annah in your party, you can find Pharod's skull in there, who only appears for a few moments while crying out to Annah for mercy. It's also where Morte came from... and you can shove him back in. You can also shove poor Annah or Grace into it, or you can go into it yourself, though not permanently — the skulls just eat you alive before spitting you back out, represented by a huge drop in maximum HP.
The shadows. They're horrific undead monstrosities that are actually the souls of people who died to balance Death's books and fuel the Nameless One's immortality. Think about how many shadows you see throughout the game, and how twisted, warped, and hateful they are. They used to be ordinary people. Also doubles as Fridge Horror.
Twice over when you realize the consequence of their hatred. Every time they kill the Nameless One, they just make the problem worse.
Ingress, who was living a happy life in her world. Then, one day, she passed between two trees while whistling a tune, and it turned out to be an one-way portal to Sigil activated by this particular tune. By the time you meet her, she's been trying to get out for a very long time, and after scarring experiences on several horrible planes she's utterly paranoid about anything that could conceivably be a portal. Now imagine all that happening to you.
Adding to the fuel, she was a little girl when she passed through the portal and when you meet her she is an adult but you can't tell which age because of the many wounds and how this life has worn her out. It is however stated she was trapped for thirty years!
Everything about playing an evil Nameless One. It isn't like in other games, where you play an "evil" character by going on bloody rampages, being selfish, or being mean in a hilarious fashion. Being evil in Planescape: Torment is just an endless depressing march of sociopathic cruelty. It has been said that the only way to be evil in this game without being so beaten down as to quit is either to skip through all the dialogue and not look at the horrible things you are doing, or to genuinely be a psychopath. A few examples:
If you obtain a particular magic tome, you can sacrifice one of your allies for power, or sell one of them into slavery for even more power. Made particularly cruel for Grace, who actually used to be a slave, and Dak'kon, whose entire race were once slaves of the illithids.
You can release Coaxmetal from the Siege Tower with the Modron Cube, simultaneously unleashing a massive force of destruction on the planes, while also damning any Modrons who were still in the Cube's pocket dimension. Don't worry, though, you get a really nice weapon out of the deal.
As stated above, you can feed one of your allies to the Pillar of Skulls in exchange for information, condemning them to eternal damnation for the sin of believing you were their friend.
When you find Trias the second time you're in Curst, you can allow him to continue on his crusade to bring the Blood War to the Heavens, dragging countless innocent souls into a cosmic bloodbath for the sake of vengeance.
Fhjull Forked-Tongue has been magically sworn to give charity to anyone who asks for it. Because of this, you can effectively rob him blind of all his most treasured possessions, while he has to grin and bear it, and after you are done, you can murder him in cold blood for good measure while he isn't even able to properly defend himself, just to rub it in; and no, it doesn't make the act any less evil just because he's a devil.
You can continue your previous incarnation's abuse of Deionarra by manipulating her into giving you aid, even lying about loving her until you shatter her hopes of reunion once more.
If you want to become an Anarchist, you first need to join the Believers of the Source, and then betray them by sabotaging their huge project, condemning everyone who works on it to death. It only becomes nastier from there, even though the Anarchists aren't technically an evil organization.
The memory in the sensory stone called "Messenger". It is a highly descriptive look at Ravel the Night Hag's cruelty through the eyes (so to speak...) of one of her victims... and she was being merciful.
Some lesser sensory stones are rather unnerving as well, though nowhere near as horrifying as Messenger. Unavoidable Pain, for instance, shows the experience of a person who, while struggling in combat with an enemy, had their hand slowly pushed into a lava flow.
In the hall of the Sensates, a faction of people dedicated to experiencing everything, is a grizzled old man named Ghysis who is a veteran of the Blood War, the eternal war between devils and demons. He's spending the rest of his days lecturing to anyone who will listen about what the Blood War is in the hopes of preventing others from making the mistake he did and signing up for a tour of service. And the stories he has to tell are truly chilling, and serve to make one of the several endings of the game even more powerful.
Making things worse, if you talk to the alu-fiend who runs the Curiosity Shoppe, she'll tell you that Ghysis only escaped by letting the above-mentioned Pillar of Skulls eat the only other two survivors of his unit in exchange for being told where to find a portal off of Baator and how to use it. Even worse, you can actually call him out on this and he'll break down in front of the whole lecture.
Really hammered home when you use Deionarra's stone in the Festhall's Private Sensorium; even the most Chaotic Evil version of The Nameless One will break down in tears In-Universe when he's confronted by both Deionarra's own feelings and the total heartless rage and cruelty that the Practical Incarnation was actually feeling when Deionarra was talking to him.
If you're banished to the Nameless One's maze by the Lady of Pain, you'll likely come across an old campsite containing a brimstone axe and an old journal made from leather and bone that was written by the Paranoid Incarnation. If you think about it too long, you realize that the Paranoid Incarnation made the journal from the only materials that were available: his own flesh and bone.