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Nightmare Fuel: Pathfinder
Lamashtu, full stop. Besides being a goddess of monsters and nightmares, there's all the horrifying details surrounding her progeny and worshipers. For example, Lamashtan priestesses who give birth to children blessed by their goddess do so by letting their offspring tear their way out of the womb.
More fun in this vein are The Motherless, The Tieflings with Qlippoth heritage. They're implied to eat their way out in childbirth.
Also Zon-Kuthon, god of pain, who preaches torture, mutilation (of both oneself and others), and dismemberment of living victims (who are kept alive as long as possible). Zon-Kuthon brutally tortured and flayed his own father until he was a broken and twisted slave.
Most of Rise of the Runelords, with a generous side order of Squick. Special mention goes to The Hook Mountain Massacre (part 3), which turned the comically bumbling D&D ogres (traditionally just sword-fodder for guys who'd mastered killing orcs but weren't ready to kill giants) into beings truly worth of the Always Chaotic Evil label.
Carrion Crown is a swift trip into the horror genre. What does it start with? A haunted prison, what else!