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Nightmare Fuel: Dragon Age: Origins – Awakening
One of your party members is a spirit of the Fade that must possess a corpse. So basically you have an animated corpse, still with its old, grave injuries that killed the body, and soulless eyes. Try not to look at him too hard.
The Architect doesn't seem that creepy at first — especially not when compared to The Mother. Then you notice the little things that seem "off" about him. That Nice Hat of his? It's actually part of his head. And his eyes...yikes.
The final part of the short sidequest "Adria's Plight." Anything can be made scarier with shrieking.
Made worse for Nathaniel, who knew her growing up and considered Adria to be like a mother to him.
Drake's Fall is even more unnerving than the Deep Roads. Imagine a desolate wasteland in the dead of night littered with the bones of long dead dragons. Then add the hordes of Darkspawn. Then add the Children. Finally there's the cherry on the terror sundae: being ambushed by a High Dragon.
At the end of the "Last of the Legion" quest, at the very bottom of Kal Hirol, you find the Broodmother pit. Just one of those things is bad enough, but three... Brrrr.
The Warden arrives just in the nick of time to save Sigrun from being carried off by darkspawn, something she mentions happened to all of the other women in her unit. Made far worse as it's implied she knows perfectly well what happens to women the darkspawn take prisoner.
The Blackmarsh questline, from the start to finish. Dark swamp with werewolves and Children lurking abound, coupled with multiple, eerie Veil tears. And in the middle of it there's an abandoned, overgrown village. The part in the Fade isn't that bad if you don't count the crypt, but when you return you have to fight a force of Revenants and Shrieks, and the less we speak about The Baroness' true form, the better.
For me the scariest part of the Blackmarsh were the Blighted Werewolves. Here are creatures already cursed, transformed into monsters and unable to control their violent rage. And then they get the darkspawn corruption, probably through eating them, leaving them twisted, mad and in constant pain. Killing them felt like doing them a favour. Plus some of them can just POOF out of nowhere and suddenly your uberpowered main (or your horribly, horribly underarmored mage) is pinned to the ground by an Overwhelm from a high-level miniboss...
The Children in their Gigerian glory are pretty damn horrifying: take a human baby, cross it with a giant maggot, giver it some sharp teeth and jagged claws, and you got yourself Children that at times during battle will grab fellow darkspawn or hapless NPCs, devour them whole and turn bigger, faster, and gooier (and more dangerous) as a result.