Sirens

I walked over the Bone Shores near Hulwar, as I watched a ship sailing past me. Not an unusual sight, but I heard... a song. Singing. Beautiful singing and I knew I was close to some Sirens which I had hoped and feared to see. Luckily those beautiful creatures had spotted the ship and lured the sailors in. I heard someone screaming, but the Sirens were as merciless as beautiful. Canons barked, swords were drawn, but the ship sunk inevitable and then there was silence.
Lucky as I was some sailors had killed a Siren, her motionless body washed upon the Bone Shore, with marks all over her body. She had a iridiscent blue-ish green tail with four small fins, permeated with thin bones. Her skin was not smooth, it was made out of small green scales, her chest looked like a female human, but her "breasts" were actually something I would call "gas bladder", because it was squishy without any other resemblance of a female breast. Allegedly for traversing the seas. Her belly was flat and something like a slit disturbed the waves of scales, colvering her whole body. I assume beneath the slit would be some reproduction organs and other internal organs, but I couldn't examine her further.
And she had tiny, sharp fangs in her mouth and sharp claws. Her hair was no human hair, it had the feeling of corals and was scratchy, slender and flexible. I cut some strands off and hurried to get to a safe distance. You never knew when a Siren might want to sing you to death. Even in death her eyes were mesmerizing and I wouldn't dare to come near a living Siren, not for all gold in the world.
From Dasoths Bestiary - Animals & Monsters around Koria

"What is she singing? It is so beautiful!"
"You... oh, you are a man. For me she is just reciting some kind of cookbook."

— A male and a female sailor at the sight of a singing siren

What is a Siren, you ask? Well, a Siren is the distant cousin of the Mermaid and the enemy of them. Both Siren and Mermaids have a female and a male and they hate each other. No, you don't understand, they hate each other.

Why do they hate each other? Because Sirens - male & female ones - try to eat sailors. Well, not directly sailors, but fresh meat. It is a delicacy and if they can lure ships into their maws, they are attempting it.

Mermaids on the other hand are friends of the surface dwellers and try to stop the Sirens which leads to an enormous amount of bloodshed and scale-tearing.

Under the Sea...

Sirens can breathe under water and over the surface, because their lungs can filter it with a complex mechanism. They have bladders to get up and down and lay eggs. They live in caves and something which looks like houses and buildings where they grow up and raise their children.

Sirens are fierce warriors. A thing which Dasoth hasn't seen. They use weapons like tridents or spears made from stone, wood or corals and are friends with sharks and eels. They don't hesitate to murder mermaids when they can.

There are only a few things Sirens fear: an Octopus and the Masters. Sirens - as well as Mermaids - tend to hide in their lairs, but they fear the evil will of a Master. It can be that a Master enslaves one, two or twenty families of Sirens to do their evil biddings and that is a thing a Siren fears the most: the loss of their freedom.

But how do they lure people into the sea? What are they singing? For the most part nonsense. They learn our languages through books and small chats with their victims and sing whatever comes to mind in their language. Which makes even a cookbook sounding like a loving lullaby.

Portrait of a Siren by CrazyEddie via Midjourney