Well it doesn't technically have a mermaid... Kinda but let's go with an FGO SfW and NSFW with yandere Kiara in her summer form and Caenis in her Rider form and Meltryllis in her lancer form with a mermaid spin of freely shifting back and forth from it
I’m assuming you meant the Servants have a ‘mermaid mode’ so let’s go with that!
Warnings: Yandere, Unhealthy Relationships, Sex, Sexual Situations, Kidnapping, Pinning, Drowning, Sex Underwater, Asphyxiation Kink, Implied Murder, Drugging
Sesshoin Kiara:
She loves turning into a mermaid, mostly because it’s how she first got your attention. You were awe struck; you let her get too close. So close she didn’t have a choice but to take you! You were just so cute. Now that you live with her, she’s less inclined to bring her tail out, but sometimes she does for old times sake. It still makes her giggle how wide your eyes got.
Sesshoin doesn’t usually use the tail during sex, unless you’re trying to get away. If you try to crawl into a corner or avoid her, she’ll use her tail to pin you down. The look of fear on your face is one of her favorites. She has no issue using her size to loom over you and take what she wants. If you beg or cry even better; she wants you to be helpless.
Caenis:
Caenis hates his mermaid mode; is makes people mistake him as a woman even more. The only good thing that comes with people being stupid is that it’s even more satisfying to kill them. You’re an exception; you had watched him murder someone and still came back to talk. Caenis thinks you’re insane, but doesn’t stop you from coming to visit. If you aren’t smart to run away, who cares if you go missing? Clearly you want to stay with him.
He loves it when you whimper, that mix of fear and pleasure all over your face. It happens most when he holds your head under the water, fingers stroking your most sensitive spots. Your fingers wrap around his wrists and he loves it; he wants you to depend on him because he feels like you control him. Before you black out Caenis takes you out of the water, listening to you gasp and struggle. He’s never going to let you go now.
Meltryllis:
She’s one of the most gorgeous mermaids in the world and Melt knows it. She flaunts it, gliding through the water and fanning her tail out to entice people into the water. Don’t trust it; the moment someone slips in, she’s on them. Blood fills the water and its horrific, in a beautiful way. You don’t meet her near the water and it’s the only way she holds any affection for you. She loves slowly, but once she wants you, you’re hers.
Melt prefers to fuck you in her mer form; her venom is stronger and it makes you numb, floating through everything. You make sure cute sounds, grinding against her tail and moaning with your mouth open. She shoves her fingers in until you choke and all she can do is coo. You’re so precious; you clearly need her to make care of you.
