This is yet another of phannniemay. The prompt was "Fave AU", and I chose Mer!Danny. More of the AU can be found in the tags on tumblr.
He was so excited! Sam, his Sam, was going to go with him to home!
He practically dragged her to the beach, hand gripped tight (but not too tight) as the two moved towards the water. Clothes were shed (his own, as they held no purpose in his homeland, while Sam insisted on wearing something called a 'wetsuit', which was neither wet nor looked anything like a suit) and the two splashed into the water.
Danny laughed as he swam, his human legs pressing together. Whatever humanoid features below the waist he'd had (of which he'd been likened to a Ken doll, whatever that was, as the transformation was primarily external rather than internal) fell away as the whole skin of his body rippled, shifting and shuddering as if everything underneath the skin had gotten just the slightest bit smaller; or as if the skin itself had gotten larger. Danny went limp, and the observing Sam froze mid-stroke.
"Danny?" It was the first time he'd gone back into the sea in months. He'd never explained how the transformation worked, nor did any of the Fenton family provide details, but now it felt like the most important information had been withheld.
Danny's entire body spasmed, twisting and curling in ways that were entirely unnatural to human physiology. Both legs pressed tight together, and something dark, green, and looking far too thick to be water seeped from between them, and Sam thought she saw a look of intense pain in his face before it too vanished within a cloud of green. Sam screamed, a terrified shriek that went unheard in the late afternoon wind.
Something big and white darted out of the green cloud of blood (for what else could it have been but blood?), and Sam twisted in the waist-deep water, trying to keep track of where it was. Whatever it was had–
"Sam?" The scream the it tore from her this time was more visceral, because now she could see it.
"D-danny?" It bore little resemblance to the merman, it's skin bone-white and hairless, the tail more like that of a shark's than a dolphin's. An arm, looking much like a cross between a fin and a bat's wing, reached out towards her, and a mouth filled with sharp little teeth grinned out at her. There was no nose, nor lips, not even eyelids to cover the too-big shining green eyes.
"Sam!" Danny (and for sure with eyes that wide and green there was no way this creature wasn't him) reached out, arm-fins slipping around her body as he pulled her down, down, down into the ocean.
She barely had time to adjust before the water started to get heavier, pressing harder and harder against her everything, and her lungs ached and screamed in pain for air, air that she couldn't get because they were going so deep so fast and the world around them was getting incrementally darker as the bottom of the ocean dropped off into blackness.
Sam didn't dare open her eyes after that, instead struggling as much as she could against Danny, who had started nuzzling her, repeatedly murmuring "Sam" into her hair until the pressure around her was too great, too much.
That day, Sam Manson disappeared into the dark of the sea, another lost victim to the Mermaids.