I picked up the Horror Realms Pathfinder source book in a Humble Bundle, and have been reading it cover to cover. This is my review of the book, chapter by chapter. Chapter 2 is called Strange far places and it's about seven different regions on Golarion that are prone to some type of horror, so I'm posting reviews as I complete each section.
Content warning: Physical pain and self harm.
Uskheart is in Nidal, and if you're familiar with Golarion lore at all you know that, on some level, this alone qualifies it as a "horror realm." Nidal is an old nation and it's a region of predominant worship of Zon-Kuthon, the god of pain. Followers of Zon-Kuthon believe, at the very least, that pain is a significant and important part of life. It's hard to argue with that. But devotees of the Midnight Lord believe that pain is a blessing and a virtue. Fanatics seek out pain, engaging in severe self-mutilation and body modification as a form of worship.
If you've ever played an adventure module set in Nidal, you know that it's a pretty dark place. In addition to Zon-Kuthon worship, it's a nation that allowed itself to be dominated by neighbouring Cheliax, and Cheliax isn't exactly a pleasant place either.
In Uskheart, though, there's an added dimension to this darkness in the form of the Shades of the Uskwood, an ancient and sinister druidic organisation. The Shades of the Uskwood have such long, slow, and convoluted schemes against Cheliax that no one in sees them as threats. As it turns out, the Shades of the Uskwood have formed an alliance with alien entities known as "the hive," as they see this force as nature's way of restoring balance to Golarion.
Unfortunately, this book contains no information about the hive, and instead points you to Horror Adventures. I gather that the hive is an all-consuming swarm-based force, a little like the swarm of Starfinder.
Whatever the hive is, there's a location in this book with a hive camp, and some significant druidic sites. Much of the horror, though, is apparently meant to be derived from the hive, and without any notion as to what that is, this section fell pretty flat for me.
Nidal itself is pretty terrifying. Uskheart, however, doesn't strike me as any more terrifying or horrific than the rest of Nidal. I guess sinister druids can be horrifying, but they don't feel like something out of a horror movie unless, maybe, you reveal them slowly throughout a story, like a sort of nature-derived slasher movie. But that's not the plot that's provided in this section, so it would be up to you. This very much feels like an off-cut from the Horror Adventures book, and unfortunately I don't own that. I don't know whether to blame Humble Bundle for selling me part of a set of books, or to blame Paizo for not just publishing one book, but I think there's obviously a little bit of a communication problem.
Aside from that, though, this is a cool setting. Nidal is bad, and if you add evil druids into the mix it's obviously going to be more bad. Horror, though? I'm not convinced.
Next up is the final section of the chapter, the Vale of Honorless Graves, and that sounds like a horror setting.