Nereids were originally relatively benign sea nymphs from Greek mythology. Role-playing’snereids have a decidedly darker tone. Likely the name has been misapplied to a mash-up of watery fey from other traditions—RPG nereids combine the murderousness of a rusalka with the vulnerable attachments of a wiła (to her feathers) or selkie (to her sealskin). For players used to pleasant nixies and aloof but manageable merfolk, the deadliness of a CR 10 poison-spraying nereid might be quite a shock! (But at least it’s an excuse to bust out the sunder/disarm rules for stealing shawls… Speaking of which, knowledge of said shawls should be a reward for any PCs who have put plenty of ranks in the appropriate Knowledge and related skill slots.)
One element of Greek nereids worth exporting to the RPG nereid is that of family connections—nereids of legend almost always have dozens of sisters and a river god father or other watery patron. So even if killing a nereid is justified, one may have to contend with any number of upset relatives…
A nereid is surprised when a female paladin surrenders to her beguiling aura. Amused and intrigued by the novelty of the experience, the nereid eventually releases the knight without her usual drowning kiss…but the fey keeps her victim’s enchanted blade. Now the paladin is in desperate straits: While lesbianism is not against the strictures of her order, surrendering her sword to one of the Faithless is, even under magical compulsion. She must get it back before her order next summons her or lose her sacred status, and her desperation is already tempting her to larcenous and murderous thoughts of which her superiors would not approve.
An adventuring party’s ally (or perhaps even one of the adventurers herself) needs help finding her cousin. This turns out to be more complicated than simply tracking a missing person: the ally is actually an undine, and her cousin is a full-blooded nereid who does not want to be found.
A village elder is assaulted by a tupilaq. If the scrimshaw construct fails the elder is nevertheless visibly shaken and soon refuses to leave the lodge. If it succeeds, a search of the headman’s hope chest reveals the reason: a carefully preserved nereid shawl wrapped in oilskin. Either way, random attacks by sea beasts and magical assassins continue—clearly someone wants the shawl back.
—Pathfinder Bestiary 2 198
I seem to recall a nereid causing trouble for the heroes of the novel Tantras.
Meanwhile, if you’re considering travel to Unther syringesin has volunteered to be your travel agent.