User with this ability either is or can transform into an merfolk (male or female), with upper body of humanoid and lower body (in taur-style) either that of a fish or cetacean. Some are able to transform completely into humanoid form to move on land, some can split their tail into legs while others have to find other ways to move there.
Merfolk are sometimes depicted as perilous creatures associated with floods, storms, shipwrecks, and drowning. In other folk traditions (or sometimes within the same tradition) they can be benevolent, bestowing boons, teaching or even falling in love with humans.
According to Japanese legends, eating the flesh of a mermaid may also grant Semi-Immortality or Immortality, and their other organs/bodily parts such as blood and liver may have similar and related effects. This has obviously unpleasant consequences.