Nereid

Nereids are beautiful female fey that dwell beneath the ocean waves. There, they fiercely defend coral reefs and all natural life that dwell within it. Although quick swimmers in their own right, nereids often seek the aid of dolphin or seahorse mounts to bolster their effectiveness in battle. Nereids are as attached to such creatures as paladins are to their steeds; should the animal fall in battle, its rider may be overcome by grief.
Each nereid wields her own personal trident, which is grown from the same coral they protect. Although they are dangerous when threatened, nereid sightings are considered to be a portent of good luck for sailors. Wise captains know to steer clear of their reefs, so as to avoid damaging them with the ship's anchor.
In addition to their skill as warriors, nereids are also well known for their exquisite voices, which can travel through water undistorted. Unlike the songs of many other aquatic fey, nereids do not weave enchantments into their music; they sing for beauty’s sake.
Benthic Beauties. Nereids are sometimes mistaken for merfolk at first glance. They possess streamlined, slightly alien features that are as striking as they are beautiful. Diaphanous webbing connects the fingers and toes of a nereid, and their slick skin resembles that of a dolphin, albeit in various shades of blue and green. Some nereids have additional fish-like traits, such as visible gills and fins that run along their limbs and back, though this aspect of their anatomy is highly variable, depending on their tribe.
Guiding the Lost. Many nereids display a level of benevolence towards humanoids that is rare among fey. Nereids have been known to shepherd lost sailors safely back to civilization, provided that they show adequate respect for nature. Some nereids even establish long-term relationships with the sailors who frequent their waters, steering their ships away from the sea’s many hidden dangers.
Defenders of Reefs. Nereids are bound to protect a specific coral reef, just as dryads are bound to their forests. Overtly hostile nereids are rare, and they usually do not discourage the intrusion of mortals into their domains. However, visitors should be wary to not mistake their relatively kind natures for softness; should their homes be at all harmed or threatened, nereids retaliate en-masse with a piranha-like tenacity, giving no quarter to their prey.