This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Yawhg
Mess up the hedge event and you'll likely want to give those thickets a wide berth for a while.
The spiral-shaped sores. If one of your heroes gets infected, then they take two major hits to their sanity, and end up destroying the next place they visit, by shattering the skulls of everyone nearby with a hideous scream. Then they dance in the blood and laugh.
To make matters worse, it's possible to get an ending where you relapse. If this happens, the best you can hope for is your protag making a Heroic Sacrifice. If not? You get to learn just what the deal is with that damned hand.
The events where the background changes are in general usually very violent and traumatizing. Be it the aforementioned sores, the werewolf that hunts you down in the forest, the vampire in the slums that is incredible hard to withstand or the bomb in the palace that you may or may not defuse/outrun. If you don't happen to be exceptional in the stats that save you, you are doomed to destroy an entire area next round, killing everyone that was there. And the best part? Even with the city being perfectly rebuild after the Yawgh, there is a 50:50 chance that the events will resurface, leading to various endings ranging from hunting down vampires in the sewers forever out of desperation for a cure up to hunting poor souls for food as a werewolf.
If the Yawhg leaves one of your heroes traumatized, the descriptions of what exactly might have brought them to that point are... no fun at all.
The Headless Raven subplot and its result after the rebuilding are intense.
While the description may verge on Black Comedy, the sub-plot that starts by pouring the potion into the river has very nightmarish results of living beings mutating into grotesque creatures that writhe in pain.