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. The final chapter, called Corruptions and haunts contains some mechanical effects for the particularly evil GM and the particularly adventurous PC.
Unfortunately, the section containing corruptions is mostly useless because this book doesn't contain the rules for corruptions, and neither does the Core Rulebook or even Game Mastery Guide. The book containing corruption rules are is Horror Adventures, and sadly the Humble Bundle package I purchased to get this book didn't include its most obvious companion volume.
I've decided to squarely blame Humble Bundle for this. Once I realised that this book expected you to own Horror Adventures, I read the front and back cover to find out whether there was any way of knowing about this dependency. Paizo makes it very clear on the back cover that this is an expansion for Horror Adventures. Humble Bundle, for whatever reason, decided it was reasonable to sell essentially part 2 of a two volume set.
It's mildly annoying, and I do mean only mildly. This is still a really good book with lots of great content. And yes, it has sold me on purchasing Horror Adventures, so this has been a successful underhanded marketing strategy (to be fair, I'm not a hard sell for buying RPG books.)
The first section of this chapter contains tables of suggested monsters according to the genre of horror you're running. I love this kind of reference, and this spans all five of the Pathfinder bestiaries. There are suggestions for body horror, cosmic horror, dark fantasy, ghost stories, gothic, psychological, and slasher. The one thing that's missing from these tables is the CR of each monster, so you do have to do a little homework in advance.
Despite not having the complete rules for how corruptions work, this section is a very fun read, and I already love the curruption mechanic. A corruption can come from one of several sources. Eleven are detailed in Horror Adventures but three are provided in this book.
There's Aboleth corruption, demonic corruption, and plagued corruption. "Corruption" isn't just a fancy name for a curse, though, and that's what makes it so fun. When you are corrupted, you gain "gifts" from the source of your corruption. For instance, with demonic enhancement, you gain a +2 profane bonus to the ability score of your choice. Not bad! However, a corruption also imposes a "stain". For instance, with demonic enhancement, you also get a -2 penalty to the ability score of the GM's choice.
There's some stuff about a "manifestation level," but those must be explained in the Horror Adventures book, because there's no information about it here.
So it's a mechanically useless section without Horror Adventures but it's still a great source of inspiration, and really sells the other book.
The Pathfinder haunt mechanic was introduced in the Game Mastery Guide. Haunts are a mix between a monster and a trap. When it's triggered, a haunt gets a surprise round to impose its effects, and players can make a check to notice the haunt. Neutralizing a haunt usually takes some combination of positive magic or even deception, and destroying the haunt often requires completing a minor quest.
The haunts listed in this section are CR 6 and above, and all have the somewhat generic theme of the First World (primordial, pre-human stuff) bleeding into our reality. This makes them ideal for really any adventure, not just horror-themed ones, and they also serve as excellent examples of haunts. The Game Mastery Guide only contains one example, and while many adventures do have haunts that you could use as reference, you'd have to find them all first. Here's a handful of interesting haunt encounters, in one place, and for games at second tier and above (which is really where haunts probably belong, as they can be pretty tricky to conquer.)
Ignoring my confusion about the intent of this book, I consider this a great resource. It may not have hit all the horror tropes I'd expected (maybe Horror Adventures does that instead), but it definitely contains the usual assortment of Paizo's relentlessly inspiring gaming content. I anticipate using several of the locations provided, and probably not just a few of the haunts. And player options are always fun, so I hope my players bring one of these horror-themed concepts to a game.
A very good book from Paizo, as usual.