Siren

"Listen to the seagulls. Follow the seagulls. Those flying rats may sound horrid and shit all over the place, though at least just like rats they are intelligent enough to scurry close to the harbours. Much unlike a siren who will be the most beautiful thing with the most angelic of voices you'll spot in the ocean. Unlike the seagull though, her song shan't take you to the safety of dry land, though to an ill becoming fate at the depth of the abyss."
-John Harlond, Sailor Extraordinaire

Sirens are arguably some of the most dangerous creatures to have ever come from the depths. Not much is known about these mysterious agents of the deep lore, but that they are part of the monstrous horde known as the Glóin.
In their natural habitat, these creatures look more akin to merfolk than humanoid with almost vampiric bat wings stretching from their arms and down all the way to where their thighs would've been, if it wasn't for their legs being shaped like a long and terrible, almost spiked tail which allows them to swim at extreme speeds, though also propel out of the water to fly for up to two to four minutes before descending.
Using some sort of transformation magic, these creatures are able to move their scales about to unfold their wings and tails, to turn them into the shapes of ordinary arms and legs draped in the finest of silken like garments. They are also perfectly able to replicate the modern fashion and general ethnic facial and body traits of the local populations they are spying on. Thus making it incredibly hard to discern a threat from an ordinary woman, at best. At worst, attempting to take out a siren could end up in a political disaster due to their charm and control of dementalistic magics, that often has the common individual fooled that they are some important noble.
Even then, as a species that will shrivel up into a dusty corpse if without a long soaking bath once a week, it's wholly unknown what exactly they want so badly on land, or why they even target sailors at sea. One concern is that they might eat the flesh of mortals, though there have been no recorded instances of it nor ever found any residue of humanoid flesh in their bellies on the autopsy reports. Hence, making their motives to even bother with the world above most puzzling.

Creature Type

Species: Siren
Family: Wilderkin, Mermideae
Class: Mortal, Glóin
Domain: Amphibious

Biology

Size: 1.6 - 1.8m
Weight: 40 - 65 kg
Diet: Omnivore
Intelligence: Likely higher than yours
Lifespan: 350 - 70 years

Habitat

Preferred biomes: The Abyss
Native regions: Unknown

Hunting knowledge

Strengths: High intelligence, capable of transmutation magics, dementalism, illusion and charms
Weaknesses: Will dry and shrivel up to a withered corpse if without drowning her body in water for three days to a week
Special Abilities: Can transform the scales of her wings and fin into a pair of humanoid legs accompanied by a beautiful and exotic dress