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Ah, the yithian (or the Great Race of Yith if you’re nasty). Even by the standards of H. P. Lovecraft’s aberrant menagerie, these guys are bizarre. Conical bodies; head, claws, and feeding tubes topping tentacle stalks; nasty mind/memory powers; even the ability to project themselves into the Astral. But it gets weirder: These odd bodies aren't even their original forms. Foreseeing that their race would be destroyed by flying polyps, yithians projected the minds of their entire species across time and space en masse, swapping mental places with the creatures that originally possessed these alien shapes. And having fled one apocalypse already, yithians continue to keep their eyes (all three of them) open, exploring planets and time periods across the galaxy, always seeking knowledge, safe refuge, and intelligence regarding their ancient enemies.
And that's really the key to these creatures. Other Lovecraftian beasts are more blood-curdlingly terrifying or will melt your sanity just by showing themselves. By those standards yithians are practically harmless, even acting as gracious hosts to those whose bodies they borrow for their research. But these are cold, calculating scientists who once doomed an entire race to extinction by mind-swapping with them—and they did so literally as quickly as thought. To creatures like these, PCs are dupes and lab rats at best, by definition utterly expendable.
Adventurers investigate reports of lightning strikes that may presage lightning elementals—or worse yet, a blue dragon. Instead they find a stretch of beach studded with copper rods. It is the lightning farm of a yithian attempting to recharge his depleted equipment and awaken a half-buried, temple-like structure. Unfortunately for the adventurers, his lightning gun (see the Occult Bestiary) is already partially recharged (for 3d12 electricity damage).
Adventurers have taken over an astral ferry service, guiding travelers from color pool to color pool as they skirt around psychic storms, shulsaga raiding parties, and the Soul Spire. During once such expedition, they find themselves being trailed by a band of outlandish figures. The creatures are astral-projected yithians, and they are curious about the adventurers’ magical skiff and what worlds (and eons) they hail from. The yithians fully intend to take a ride in the adventurers’ bodies; how politely they ask (or not) is an open question.
The fortress palace of Chasmhold is famous for the oubliette in the center of its throne room—a seemingly bottomless hole that enemies of the state are lowered (or hurled) into and forgotten. In reality, the oubliette does indeed have a bottom (carefully concealed and silenced with illusions and other effects). But that deep dark cellar floor is merely a plug over a larger, deeper hole...one that hides a deadly flying polyp. And an apostate yithian has decided the best revenge upon the race that exiled him is to wake the ancient enemy…
—Pathfinder Bestiary 3 286
Edit: Original post: The best-laid plans of mice and men... Just got off the plane from Florida; will update ASAP tomorrow. Thanks for your patience.