How do you get the party together? That’s always the big question, isn’t it? Especially when your group is a ranger with no name and no friends who needs no one, a rogue who plans on stealing from the other PCs like it’s 1978, a barbarian who hates wizards, the wizard that barbarian hates, and a samurai?
Easy: You use the ningyo.
In Japanese myth, the ningyo is a kind of monkey-mermaid whose flesh is credited with remarkable longevity…if you’re willing to risk the possible curse or storm some tales promise. The Pathfinder version has tweaked this origin by sort of it turning inside out: namely, a slain ningyo returns to life as an undead creature at night.
Did you catch that? A monkey-mermaid that comes back after death as a nocturnal zombie. Oh, and its flesh is poison. And it gets a bonus to swarm and grapple in groups. And it’s very startling. And did I mention THE ZOMBIE THING?!?
When you’ve got a monster like a ningyo, adventure comes to you. It doesn’t matter how divided or unlikely the PCs are…all you have to do is get them in the same port city where some unlucky fisherman has caught a shoal of the things. Maybe they’re on the docks or the fish market when the things get loose and attack. After the ningyo are slain, maybe their bodies are claimed as trophies or bought as curiosities and spread all throughout the city. And when night falls—bam, instant encounter. The ranger who needs no one will learn his lesson when four of the things jump him. The barbarian might loosen up when the wizard’s magic missile saves his life. The rogue will learn a lesson about stealing when his catch of the day animates and attacks him. The samurai might be eyeing a ningyo under glass in his lord’s palace or be a penniless ronin doing chores for fish scraps—it won’t matter once the undead mer-monkeys come alive and attack him. And when all the characters arrive at the docks the next morning to find out where the hell these things came from, they’ll all have at least this one thing in common. Party formed.
When a hurricane drives a shoal of ningyos into far northern waters, a troublemaking sea pixie sees an opportunity for mayhem and lures the creatures into fishermen’s nets. Soon curious collectors and scholars spread the creatures—or their corpses—throughout the city. The pixie is thrilled at the resulting chaos, and will be quite perturbed at any do-gooders who attempt to spoil his joke. While he won't attack, he’ll use the magical tricks at his disposal to make rounding up the undead ningyo as difficult as possible.
Manowar, the Drowning City, lies half-sunk under water. Adventurers hired to find a lost child need to traverse the knee-deep (and worse) water of the Lower Quarter to continue their search, braving the cramped tunnels, armor-ruining salt water, and the jellyfish that give Manowar one of its names. In the process, they unwittingly wander into a turf—or rather, surf—war between the local ningyos and ravenous grindylow invaders…a war that only gets worse at night.
The Fane of Innis will fall if any undead breaches its hallowed nave. So says the prophecy at least. But as the goddess herself has blessed the temple and its surroundings, undead are repulsed from the holy structure long before they could ever violate its central chamber, and no divine or arcane spell of animation will work on the temple grounds. Her light does no harm to corpses, however…and one very determined and well-traveled apostate has returned from the Saffron Isles bearing a water jug with an extremely unusual inhabitant…
—Pathfinder Adventure Path #37 80–81 & Pathfinder Bestiary 4 206
Oh, what’s that? Japanese monsters don’t fit with your European-style campaign? Ignore the monkey part and call her a shark sprite or a needletooth or something. But you already knew that—I’m just using this paragraph as an excuse to link to my favorite mini-mermaid of all time, courtesy of Nina Kiriki Hoffman’s “Out of Hand” from Dragon Magazine #132.
Edit: For two more ningyo adventure seeds, see the original entry in Pathfinder Adventure Path #37: Souls for Smuggler’s Shiv.