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 1 is called Searchers after horror and it's all about player options. The first section of the chapter acknowledges that characters inspired by horror may not integrate naturally into a traditional fantasy seting. This is significant because Pathfinder's setting, the world of Golarion, tends to be an all-inclusive setting. It does have different planes of existence and it does have a solar system, but as a gamer you don't go out and purchase separate source books to explore different settings so much as you just travel to some other region of Golarion. You want gothic horror? Go to the far off land of Ustalav. You want space age technology? Go to Numeria. And so on.
So the first section of this chapter encourages players to recognise the uniqueness of their horror-themed character, and work to justify it in whatever setting their game is set in. It might seem obvious to many players, but this section is a good reminder to try to keep in mind that Pathfinder is a roleplaying game.
Some Pathfinder classes are practically designed for horror as it is. The Witch, the Spiritualist, the Mesmerist, and the Inquisitor classes all fit easily into a horror setting, but this book apparently feels that the Bard and the Arcanist could use some new options.
And when Paizo advertises horror themed, they do mean horror themed.
For the Arcanist, for instance, there's an ability called Blood tears, in which you can spend 1 arcane point to bleed from the eyes. This protects you from gaze attacks without causing your enemies to gain concealment from you. You can then wipe the tears from your eyes and flick them at your enemies for a chance to stagger them for 1 minute.
Fiendish proboscis causes you to grow a proboscis out of your mouth. You can attack with this proboscis, and even drain magic from a magic user to restore your own arcane reservoir.
The Bard class gains options for damaging performances, some of which feed on your own Constitution.
These player options are the closest I've seen to some of the actual character flaws used in Shadowrun. They aren't really that bad (Shadowrun has charater traits you can take that actively hinder you in some way), but they read pretty horrific. This is some dark stuff.
Did I mention this was a dark chapter? This section is for any class with an animal companion (Druid, Ranger, Witch, probably others that I'm not thinking of off the top of my head.) The section on accursed companions introduces the concept that maybe your animal companion died and came back as essentially your own personal haunting. There are other ways to theme it. Maybe your animal companion was tortured by your enemies before it was killed, or maybe you just didn't show it enough love and affection before it departed the material plane. Whatever happened, your animal companion is accursed, and gains features that help it and hinder you.
For instance, the Bloodthirsty ability causes your companion to go into a rage, gaining +2 CON and +2 STR. Sounds great, except that your character is reduced to doing nothing but growling, hissing, and shrieking. You're unable to speak coherently, much less use spells with verbal components. This lasts for 1 minute unless you make a succsessful Will save, in which case it lasts only 1 round.
The Festering flesh ability causes your companion to appear as rotting and festering and infested. Your companion is more affectionate than ever, though, so it's always close by. You're immune to the sickened and nauseated conditions, but you also suffer -2 on saving throws against disease effects.
This is an archetype for monks. I'll refrain from descriptions of this section, because some of it involves self mutilation.
I personally have no problem with this topic in fiction, so I will say that I loved this section. It's dark. It's some degree of horrific, depending on your tolerance. It's definitely a variant of the monk class I would play.
This section lists a collection of curses for the Oracle class. It says that when a player character acquires a curse, it's the Game Master's choice to make that character "susceptible to gaining the corruption of the same name."
It took me several minutes to unravel what that actually meant. I searched through the rest of the chapter but found nothing about "corruptions." I did a quick search through the rest of the book. I turned to the Core Rulebook, thinking maybe there was a mechanic I'd never encountered in all the years I've played the game. Well, according to the Internet, the Corruption mechanic is something introduced in the Horror Adventures book, which I don't own.
I assume these curses are standard curses, whether or not you use the Corruption rules or not. They're pretty harsh, so hopefully you've got a party member with Remove curse. But then again, they also have bonuses so maybe you want to live with the curse.
Never before have penalties in Pathfinder felt so good.
This provides class abilities to transform the Shaman class into a Lovecraftian cultist. You dabble with the powers of the Outer Gods. Strictly speaking, I don't think that would go well in pure Lovecraft, but I guess as long as you're just dabbling you get to retain your faculties. The Brain drain ability allows you to violently probe the mind of your enemy.
Once you get into higher levels, you gain the ability to contact another plane where an alien mind dwells. Fail the save against your ally and your INT and CHA are reduced to 8 for 5 weeks. I told you it wouldn't end well!
Horrific creatures to summon. When playing the Summoner class and you choose to summon this creature, you get to build your own variant of the aberration. To build an aberrant eidolon, you use evolution points to buy specific abilities and powers. It's psychic, psionic, and Lovecraftian, and it's really really cool.
This chapter was a clear departure from the classic gothic horror tropes, and yet it's not quite pure Lovecraft or Stephen King or Clive Barker. It's an oddly natural blend of all the horror you can think of, and it doesn't hold back. I love that there are penalties mixed in with the bonuses. It makes me think fondly of Shadowrun, and it feels fairly unique in a D&D book. This chapter has sold me on this book, without question, and I can't wait to find out what's next.