Okay, it’s time for me to gush about the hyakume. Ready? Ahem:
How cool is this thing?!?
A corpulent blob of eyes and fat rolls strolling down the nighttime streets? That sends disembodied floating eyes to follow characters around? Floating eyes that can even drain memories? (Not to be crude, but that sound you just heard off in the distance was Ed Greenwood having what the Brits term “a crisis.”) And while it prefers to work from the shadows, should PCs corner one it can fell them with a blast of ice, sound, or a monk’s quivering palm attack. How is this not in your campaign already?!?
(Well, maybe it is. But it’s never been in a campaign I’ve been in, and it needs to be.)
And what is a hyakume enclave like? Are they organized crime lords? Mystic evil friars? The secret masters of the city? Pathfinder doesn’t have beholders or mind flayers—are hyakumes the replacements we’ve been looking for? All signs point to “Yes.” (As creepy as neothelids/seugathi are, their lack of humanoid aspect holds them back somewhat in the ol’ “looks evocative” department. And urdefhans’ habit of worshipping oblivion is always going to make their recruitment efforts difficult.)
If I have one quibble with hyakumes, it’s that they’re slightly too powerful for many campaigns—if they were CR 10 or CR 12, you could give them more face time with PCs and tweak them more easily with class levels and whatnot. But that’s like complaining that the girl from Girl with a Pearl Earring should have thought about gauges instead. These blobs of fat and eyeballs are one part Miyazaki fairy tale and one part body horror manga, and I love every ounce of them.
The Monster Parade is famous throughout the four realms. Occurring once a season, always on the night of a new moon, kappas, fey, and yokai of all sorts march through the streets of Tomori, bearing lanterns to keep the dark away. The strange sight is beloved by locals and travelers alike as a gift from the Underworld. In reality, the tradition is a brilliant scheme by one of the stars of the Parade, the hyakume Father Eyes. Thanks to the Parade he is able to hide in plain sight, sending his eyes off on mysterious missions and memorizing the faces of future memory victims.
A hyakume keeps oni and other fell spirits out of his city in the most efficient way possible: devil retainers, bound into service through carefully collected memories of truenames, words of binding, and the truths behind old betrayals. But when one of his devils breaks free of his control, to be followed by reports of demons and an atamahuta (see Pathfinder Adventure Path #51: The Hungry Storm) running loose in the Lower Dens, the hyakume knows something must be done. Through intermediaries he recruits adventurers to clean up the errant outsiders. But when it turns out the incidents are all connected in one plot, the adventurers inevitably discover who the target of the plot was. While grateful for their help, the hyakume nevertheless deems it prudent to have them killed to secure their silence.
An enclave of hyakumes runs the Evanescent Repository, a library for memories. So respected is the collection that even viduus psychopomps and bishop agathions visit to consult particularly fine recollections. The hyakume are shy nocturnal creatures that try to avoid conflict, but in their greed for memories they will occasionally abduct and even murder to claim a thought they need. Adventurers who try to confront the enclave will have to defeat the Repository’s many guardians, talk or fight their way past the visiting scholars, and discern the hyakume librarians from the corpulent, many-eyed roper guardians who look almost indistinguishable from their hyakume masters.
—Pathfinder Bestiary 4 153