My favourite parts of Pathfinder lore mostly involve the Shadow Plane. From the kayals & shae to the corruption by Zon-Kuthon & terrain and so on. In working on my characters and game/writing ideas, I've been digging further into and trying to look into similar settings for ideas which is why I'm posting.
I know D&D has Shadowfell and there are plenty of myths of Underworlds and shadowy places. I also know many of the other non-human races we can trace through so many other stories. I guess I'm trying to do that with some of the Shadow Plane races and spaces and hoping others might enjoy the shadowy side as much as I do and can recommend some stories and resources.