Nixies are a species of aquatic fey originating from Germanic mythology, essentially freshwater-dwelling merfolk who could shapeshift into animals and who often lured mortals into the deep - to drown them in grimmer tales, or to keep them prisoner in their underwater lairs in more light-hearted ones. Similar creatures can be found in Scandinavian and English mythology as well, although the general idea of being river and lake faeries remains consistent.
Nixies resemble somewhat attractive humanoids with green skin and hair and webbed appendages. Nixies are goodly and peaceful creatures, but are shy, reclusive and suspicious. Nixies desire friendship, and have the ability to charm others to become their friend. Nixies love music, and make instruments from reeds on the banks of streams.
The nixie first appeared in the original Dungeons & Dragons set (1974). The nixie appeared in first edition Advanced Dungeons & Dragons in the original Monster Manual (1977). The nixie appeared in the D&D Expert Set (1981, 1983) and the Dungeons & Dragons Rules Cyclopedia (1991). The nixie appeared as a player character class in The Sea People (1990). The nixie appeared in second edition Advanced Dungeons & Dragons in Monstrous Compendium Volume One (1989), and reprinted in the Monstrous Manual (1993) under the "sprite" entry. The nixie appeared in the third edition under the "sprite" entry in the Monster Manual (2000), and in the 3.5 revised Monster Manual (2003). The nixie appeared as a player character race in Savage Species (2003).
A nixie is a small, aquatic fey creature that makes its home in lakes, rivers, and ponds. They often serve as the de facto guardian of their watery homes. A casual glance might cause them to be confused for an elven child, thanks to their proportions and features (long pointy ears, narrow chins, small mouths and big eyes), but a closer look reveals their obvious inhuman traits. A nixie's skin is covered in very, very fine, green-tinted, fish-like scales, its hair resembles river weeds in its texture and greenish color, and its hands and feet are both heavily webbed - they more closely resemble a duck's foot or a fish's fin than a human appendage. Nixies normally stand about four feet tall and weigh around fifty pounds.
Nixies can dwell in any body of fresh water ranging from ponds and streams to mighty rivers and huge lakes. Their only preference is for their home to be relatively isolated from civilization. Even when far from other intelligent races, nixies still try to conceal their homes, going to great length to remain hidden. Nixies, while sometimes found alone, often dwell in tribes of up to eighty creatures, although only in the largest lakes and rivers. They prove tenacious guardians of their homes, normally using trickery and deceit to get rid of intruders, but are not above resorting to violence if such methods prove ineffective. They are also known to charm and enslave humanoids to be their guardians or to perform tasks that require greater strength.
It's worth noting that while most nixies are shy, peaceful creatures, there are some types of nixie that are not. These nixies, commonly known as "bog nixies" for their preference for dwelling dismal fens and gloomy swamps, delight in luring land creatures to their aquatic home and drowning them. They lure their prey by using their incredible beauty, then watch them drown as they enter their underwater home. These nixies also possess the ability to change their form into that of any aquatic creature, and can even grant minor wishes - which, ironically, serve as a lure for the foolish, overly bold or desperate, and gives them a greater opportunity to work their mischief.
The other kind of Nixie, most likely to come up if you're running a real-world-Cold-War setting or campaign or you know some really weird nerds.