ANYTHING to do with the King of Shadow's back-story, as fleshed out in the Ruins of Arvahn. The days of agony he spent being turned into a giant magical construct while his lover watched, what happened after he was shunted to the Shadow Weave, and even the final confrontation with him shows just how much there's nothing left but an automaton.
Horrors committed by the King of Shadows post-Netheril also include: ripping apart six women's souls and merging them into one creature, imprisoning a dragon that fought with the empire he was supposed to protect, countless wars with other groups like the Gith, and of course, everything in the campaign like the turning the Mere of Dead Men into a wasteland. Not to mention the Bad End that happens if you ally with him.
The massacre of Ember. Just the thought that someone would cross that line just to frame you for it. And later when you investigate the crime scene, try read the description on how they were killed. Some examples: cats being cut in half, one man being left behind to bleed to death and a elderly couple being brutally bashed in the head, having their skulls crack!
Mask of the Betrayer
The expansion is a massive tanker with Nightmare Fuel, but let’s pick the largest barrels.
The Wall of the Faithless: A massive structure composed of souls of Nay-Theists in the Forgotten Realms setting as punishment for not worshipping any deities. They’re slowly petrified out of existence in a very painful and horrifying manner. This doubles with Fridge Horror, because when you finally see it, it’s very tall and thick and it surrounds an entire planar metropolis. It’s only been around for a few centuries and it only counts for Nay-Theists in Abeir Toril. A world where the gods existence is obvious. Who are everyone in the wall?
It gets worse. During your dreamscape encounter with the wall, you'll even meet Bishop who's trapped in there. And during your conversation the wall starts growing and you'll hear several souls screaming in unison. You can even hear kids scream! That's right, not even kids are safe from the wall!
The Spirit Eater curse: A Body Surfingabomination that force the host to devour souls, erasing them from existence. If the victim doesn’t get sustenance, he/she will go mad and eventually die and the Spirit Eater will jump to the closest victim. Oh yeah, and their souls are transported to the aforementioned wall once they’re cursed.
The Spirit Eater's appearance is pretty messed up. It's a floating, black husk with a huge maw and several flailing tentacles. And keep in mind that it was originally a man, turned into an entity of pure hunger as punishment for rebelling against his god.
The beginning. After your fight against the King of Shadows, you wake up in a barrow, far away from the Sword Coast and your friends that could be dead for all you know, filled with powerful spirits bent on keeping you trapped. The silver shard you've had within you since infancy is crudely removed and there's something within you that suddenly forces you to eat a spirit that you had just spoken to.
That... thing in the water outside the Sleeping Coven. You can see several huge tentacles petruding all over the lake and each is marked as Impossible Difficulty. Fortunately it won’t do anything to you, but, what exactly is that thing?
Gulk'aush’s ravings inside the Sleeping Coven. It can surprise you the first time and you’ll have no idea at first who it is and they’re not pleasant to listen to.
The conversation with Myrkul on the Astral Plane where he’s Mind Screwing you and the music doesn’t help much.
His backstory and involvement to the plot is downright terrifying. It’s unnerving how evil someone can be, and you can potentially be just as bad in the ending if you embrace the curse.
