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Nightmare Fuel: The Elder Scrolls III: Morrowind
The ancestral tombs. It's not enough that they're all cramped dungeons, but then you hear the ancestral spirits whisper. Then you find yourself brutally surprised by one of the various permanently grinning undead.
Dagoth Ur is literally nightmare fuel, he is a Mad Dreamer who gives Bad Dreams to people. Here's a quick example:
I AM THE SHARMAT I AM OLDER THAN MUSIC WHAT I BRING IS LIGHT WHAT I BRING IS A STAR WHAT I BRING IS AN ANCIENT SEA WHEN YOU SLEEP YOU SEE ME DANCING AT THE CORE IT IS NOT A BLIGHT IT IS MY HOUSE I PUT A STAR INTO THE WORLD'S MOUTH TO MURDER IT TEAR DOWN THE PYLONS MY BLIND FISH SWIM IN THE NEW PHLOGISTON TEAR DOWN THE PYLONS MY DEAF MOONS SING AND BURN AND ORBIT ME I AM OLDER THAN MUSIC WHAT I BRING IS LIGHT WHAT I BRING IS A STAR WHAT I BRING IS AN ANCIENT SEA
Corprus disease is And I Must Scream in a contagious form. It's similar to leprosy or cancer... but worse. Corprus twists your mind and body the longer you have it, sapping your intelligence and driving you mad. Like leprosy, it is obvious and causes almost everyone to wish that you'd go away. You will eventually become a shambling, tumorous mass as the disease takes effect; the perpetual agony will leave you Always Chaotic Evil, lashing out mindlessly at anything that comes near - and potentially spreading the disease to them. It also stops the aging process, so if you catch it and don't die of it, you'll live until somebody kills you. And you won't have the mental faculties to commit suicide any more. All you will know is the pain - and the will of Dagoth Ur. There is no known cure.