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.
Because I started reading Van Richten's Guide to Ravenloft recently, I figured I'd also read Horror Realms for Pathfinder so I could compare the two. It may not be a suitable comparison, but from a quick glance at the table of contents there appear to be parallels. There's no reason for comparing them, either. They weren't release anywhere near each other, and I have no reason to believe they were influenced by one another. However, they both talk about the horror genre in a D&D game, and so I may as well read them in tandem.
The introduction, such as it is, to this book is entirely in-world. It's a treatise called The pattern of killers by scholar Rifza Dilatru. But wait, the plot thickens. Rifza Dilatru is missing, while investigating a mysterious link he believes exists between all murders. The treatise found in the introduction was found among his belongings, left at Rust Dragon Inn in Sandpoint.
What an opener. This is one of the many reasons Paizo has the loyal following it has. They're not just master storytellers, but artists of presentation.
We'll probably never find out what happened to Rifza Dilatru (or maybe it's a plot point in one what must be hundreds of Paizo adventures I've yet to read) but what a great seed for a game, and what a great way to set the mood for what follows.
I noticed that the introductory treatise, somewhat amusingly, does take the time to define "killer." The first thing I thought when I read the title was whether or not player characters who, more often than not, necessarily slay hundreds of creatures on their journey to level 20, The treatise is sure to point out that it's interested only in psychopathic murder, not in everyday killing for self-preservation or to eliminate evil monsters.
I've been a Paizo customer long enough to know that the real introduction is on the back cover.
Horror Realms helps bring the spine-chilling terrors presented in Pathfinder RPG Horror Adventures to the Inner Sea region and beyond, presenting new rules, detailed ghastly locations, and unnerving character options for your campaign.
This is a book of player options, setting information, and special mechanics. It's roughly what I would expect from a Paizo book, and Paizo rarely disappoints.
Apparently there's a book called Horror Adventures, which I don't [yet?] own. I do own a possibly related book called Classic Horror Revisited, which details several monstrous races in great detail, but there doesn't appear to be a mention of that volume so far.
The first chapter is all about player options. With the wealth of classes that Paizo provides for Pathfinder, I can't begin to guess which ones are getting horror-based variants. The more I think about D&D's relationship with horror, the more I realize that actually a game about undead, aberrations, demons and devils, magic and the occult, death, torture, destruction, and disease, has more horror in it than one might think. What's left to make the game more horrific? Well, I guess I'll find out.