The most recent Bundle of Holding offering is Tribe8, which hands down has to be one of my favorite far too deep for it’s own good roleplaying game settings — which I read passionately but never played. Strange post-apocalyptic fantasy with humans living under the harsh protective rule of earth goddesses called Fatimas and surrounded by alien demon Z’bri horrors who wish to enslave them all once again.
From a simple inspiration perspective, the society of Tribe8 could very likely serve as great inspiration for those of the Dagger Isles. The Z’bri are definitely more viscerally horrific entities than the more elementally aligned demons of BitD but still potentially useful to read — especially if you want your demons to be darker than what it says in BitD. The Serfs of the Z’bri are good inspiration for the demon influenced people from the far off land of Tycheros.
Taking it one step further, it’s easy to imagine that humans may have survived through more than one apocalypse. The first apocalypse resulted in the world of Tribe8 with demons enslaving humans as serfs and strange goddesses protecting the humans. The Immortal Emperor could have been one of the fallen 8th Tribe who mastered a mixing of Synthesis magic and Sundering allowing them the ability to bind the demons and imprison them. This was the start of war between the 8the Tribe, the Z’bri and the Fatimas. At the height of the conflict, the mysterious Melanis of the Z’bri attempted to do what they always dreamed break open a door back to their land. This severing of walls of reality broke the Gates of Death and caused the Cataclysm, leading to the world of Blades in the Dark.