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: Mold and fungus.
Kalva is in the Land of the Linnorm Kings, a frigid northert region home to the Viking-like Ulfen people. The island of Kalva is located south of Icemark and, as it turns out, is the very embodiment of a biological horror story. Because this is a Paizo book, there are indeed mechanics to the terrifying story of Kalva and so I won't spoil too much here. Suffice it to say, though, that Kalva is overrun with molds and fungii from another realm, and that the people of Kalva are perfectly happy with their alien overlords.
Most of the horror in Kalva is derived from mold and fungus, so I would say that the predominant horror genre here is body horror. If you have players who find that kind of thing too unpleasant for a game that's meant to be fun, I think it would be relatively simple to re-theme the enemy as magical crystals or some other form of magical energy. You could even turn it into psychological horror by having the problem be opt-in. Maybe everyone in town is voluntarily indulging in the smoking of a specific herb. It seems harmless at first, and everyone in town is pushing it onto the players. Once it's been tried, you could use addiction mechanics from the Game Mastery Guide to force it onto players, and of course, it turns out to have dark and terrifying repercussions when abused. None of the mechanical effects rely on the substance of mold and fungus.
The island has other interesting features, like the Crow Witch Tribe, a linnorm (a dragon-like wyrm) called Liefbrenner, and a ghost skald named Riborg who haunts an ancient offshore outpost. Oh, and did I mention that the Ulfens of Kalva are proud cannibals?
Look, I felt compelled to put a content warning at the top of this post to warn readers about icky fungus. Cannibalism is a normal and fiercely protected way of life on Kalva. There may or may not be an entity from the outer realms (or the Abyss, or who knows where) influencing the culture of the island. This is truly unsettling horror. I have to be honest, I probably wouldn't run a game here. It's just too gross. Well done, Paizo, and no thank you!