Alien creatures who may have destroyed the Great Race of Yith hundreds of millions of years ago. Nauseating towers of flesh, eyes, mouths, and tendrils, over 30 feet tall, writhing in the air, each surrounded by a strange howling vortex of sucking wind.
In the Act III Prologue, the party experienced a series of visions from Xhamen-Dor's impossibly long lifespan, which included flashes of the Flying Polyps’ efforts on behalf of Hastur and Carcosa. They were the creators and propagators of Star Stelae across the universe.
In Chapter 173, the Prisoner sent a vision that told the tale of the Flying Polyp’s arrival on Golarion. In a desert in the southern hemisphere, they built three Star Stelae in a city they carved out of solid basalt. In the northern hemisphere, they built three Star Stele on the shore of Lake Encarthan, where Thrushmoor would eventually exist, and began to build three more on what would come to be known as Briarstone Isle, before being thwarted by an army of ancient Serpentfolk. Then the Flying Polyps found the Yithians living on Golarion, enacted a genocide of the Great Race, and enslaved the survivors.
In Chapter 175, the Prisoner sent a vision that showed the Yithians rising up against the Flying Polyps. After a deadly war, the Polyps were all either slain or sealed into subterranean tunnels below Neruzavin. Or so the Yithians thought. It turned out that a small number of Flying Polyps had escaped Golarion, and the Prisioner shared in another vision how they were later attacked by the Yithians on a different world… a world where they had been cultivating Xhamen-Dor.
In Chapter 185, the Prisoner sent a vision that showed how twenty Yithians were abandoned by their kind, left on Golarion to guard the wards that kept the Flying Polyps trapped below the earth. But after many ages, it now seems that the wards have been breached.
First Reference: Chapter 86
Other Notable References: Act III Prologue, Chapter 170, Chapter 173, Chapter 175, Chapter 185