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.
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.