The qallupilluk is another monster that manages to capture the uniqueness of its folkloric source material and still be awesome at the table.
How? Easy: the Curse of Scales (Su) ability. If it grapples you for three straight rounds, make a Fortitude save or turn into an aquatic monster devoted to the monstrous humanoid.
It gets better. If the qallupilluk succeeds in a certain dangerous ritual, the change is permanent.
That’s at once both magical from a storytelling perspective and totally terrifying at the gaming table. Being grappled always sucks (that’s why my eldritch knight is a fan of flying), but the risk of transformation raises the stakes almost as badly as being swallowed whole does. If a PC gets transformed, the rest of the party has the tactical problem of defeating the ravening reefclaw or bunyip in their midst without killing him, and of detaining the monster so the victim isn’t underwater when the change wears off. And if the qallupilluk lures the PC away, suddenly what was an ordinary encounter becomes a ticking clock adventure, because the sea witch only needs 10 minutes and 30 hit points to make a permanent slave out of her besotted servitor.
Other things to note: Qallupilluks have got the usual hag-like mix of enchantment, divination, and nature spells to marshal. They're child stealers, so defeating one is pretty much always a good thing. They treat their transformed slaves as children and then lovers, which is just icky (and ichthy). And in their original description (from Pathfinder Adventure Path #51: The Hungry Storm), they hate mirrors—a nice folkloric hook for drawing out or enraging a qallupilluk—but they will honor bargains made in good faith.
All in all, a really creepy monster, and one that works perfectly well outside its arctic setting. If you think the qallupilluk is terrifying in the frozen North, imagine one in the service of Dagon beneath a populated city…
Adventurers come across a strange festival where the locals are breaking bread with a truly monstrous fish-woman. Their arrival—and the insultingly reflective surfaces of their shiny metal gear—sends the beast to flight, and it attempts to snatch a child or two along the way. The yearly festival is part of a long-standing agreement between the qallupilluk and the igloo-dwelling villagers—so long as she is invited, she searches elsewhere for her mates. But the presence of outsiders is a breach of the contract, and now the villagers blame the adventurers, not the monster, for the kidnapping.
The magic of the ice elves is failing. Inside their ice-crystal domes, their weather magic has grown erratic. Beneath, they face assaults from tunneling saltwater merrows, reefclaws, and scrags that pop up through the ice to hunt and slay. Some desperate elves are willing to do anything to save their cities, including negotiating with qallupilluks who can both protect their fragile borders and peer into the gloomy future…even if it means giving up their children.
A town has long been in the thrall of cultists of Dagon. Subtle signs are everywhere, from the number of children born as gillmen to the secret dancing circles that spring up on stormy nights. Some of these are led by a qallupilluk, and the townsfolk regard an hour spent dancing in the form of a reefclaw or a grindylow as a sign of Dagon’s favor. They are also eager to offer up strangers to the monstrous humanoid—be they males or meals—and these transformations are not temporary…
—Pathfinder Adventure Path #51 88–89 & Pathfinder Bestiary 4 224