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.
The region of Geb is know as the domain of the dead, and its entire population are undead victims of a long magical war between Geb and Nex. Its ruler is the wizard Geb himself, although he's isolated himself to such an extent that the true ruler is the corrupted former demigoddess Arazni, raised unwillingly by Geb as a lich. How's that for a macabre introduction?
The genres listed for Geb in this book are body horror, dark fantasy, ghost story, and gothic horror. I'm not sure about body horror, but with all the undead and necromancy, it definitely qualifies for all the others. Living beings aren't forbidden in Geb, although they aren't exactly welcome either. They're officially protected by the Dead Laws, but I can imagine that a group of player characters going into Geb to steal an important item could run into formidable resistance.
I can also imagine a rising anti-living sentiment, a little like Eox's Corpse Fleet in Starfinder, causing concern. Whatever the reason, Geb could serve easily inspire tension for the living visitor. Alternately, the living could be genuinely welcome guests, brought in to solve a haunting or to ease a restless spirit.
This section provides a little background on Geb (more can be found in Inner Seas World Guide), but most notably it provides some interesting NPCs and a whole island called Crabfield Island where the living are welcome. There are several locations detailed here, including the Yellow Mire, the Golder Garden (a sinking mansion from long ago), the gemstone mines of the Iron Chelae mountains, and the trading town of Crabfield Point. The island is governed by a living arcanist ranger named Arabin Solswith, but there's lots of undead across the small isle, including the undead elf Liara Tepteki, deathspore zombies, gold musk zombies, and pale-puff musk zombies.
This is a natural horror setting. It's Army of Darkness after the Necronomicon has been removed from its resting place. Lots of undead and horrors from beyond the grave, peaceful enough to mind to their own nation but just disdainful enough of the living to make you wonder. There are some good locations on Crabfield Island, and a few good NPCs to add personality to an adventure.