Under the sea / merfolk swim free / Paizo’s are neutral / Lacking in scruples / Take it from me…
I…should probably go to Hell for that.
Anyway, merfolk’s brief mentions in the first few issues of Pathfinder were another early indication of Paizo’s project of reimagining iconic monsters. Unlike the benign merfolk of most editions of D&D, the Bestiary’s merfolk are reclusive, insular, and prone to violence—much closer to the fickle temptresses and shipwreckers of legend than Disney’s Ariel.
To me, one of the most interesting things about merfolk is the range of bonus languages they speak. This list (“Aboleth, Aklo, Draconic, Elven, Sahuagin, and Sylvan,” per the Bestiary) suggests a vast array of subcultures, influences, and threats found under the sea. Merfolk speaking Aboleth or Aklo might be in thrall to dark powers; those speaking Sylvan might be river spirits or have ties to Faerie or the First World; those speaking Draconic might be vassals of a great dragon empire or fierce drake-hating knights, etc. PCs could soon learn that the people of the sea are no less unified than the races of humanity, and a party that is a hero to one merfolk shoal may find themselves the bitter enemies of another in no time.
A capricious lorelei (a river mermaid specializing in enchantment spells or bardic charms) can be viewed from the Severed Bridge. She purposefully tries to attract swimmers to her so they might run afoul of the river’s many tatzylwyrms. She herself is a prisoner, though, bound by a witch who stole a lock of her hair. If provided proof the witch is dead, she will leave the bridge and its swimmers in peace.
Sailors used to the benevolent merfolk of the Starry Sea are unprepared for the orange-finned merfolk of the Southern Ocean. These Aklo-speaking merfolk worship demons and qlippoths, coaxing coral reefs to grow into the sigils of their demonic patrons. Even their dolphins are strange—druids will note their pronounced underbites and their tendency to click and whistle their whalespeech in an odd, affectless manner.
Merfolk samurai needing aid appeal to the courts of the Kingdom of Florin, whose queen (thanks to her family’s ties to an undine several generations back) is technically the Empress of Two Oceans. The adventurers she sends to accompany the envoys back may have to contend with rival merfolk, adaro raiding parties, and a riddling sea dragon.
—Pathfinder Bestiary 204
More on merfolk is of course sprinkled through a number of products; I’m partial to PC3 The Sea People by Jim Bambra from D&D’s Creature Crucible series.