The mermaids see you looking at them and shout in your direction:"Ooh, it's a human! Wanna come play with us and swim between the coral reefs in the depths of the ocean?" Although odd this request may seem, you are happy to oblige and move closer to them. "Won't I drown if I go that far?", you ask. The answer is that they'll "take care of it". Oh boy, you think, they're probably gonna make me a merman. "We will form your body into a shape suitable for swimming", the mermaids continue as you already feeling a slight tingle in the base of your spine.
A small mound of flesh starts to come out, quickly growing bigger and stretching outwards. "Better step into the water, human, because soon you won't be able to walk anymore." You see their point and move into the chilly sea water. The mound of flesh has now almost grown into a full sized tail, and you lift it out of the water to have a look. Strangely, it doesn't have the green scaly texture you'd expect a merman to have on his tail. Instead, it's a smooth gray, a little lighter on the bottom. "Um, since when do mermen have smooth gray tails?", you cautiously ask. "They don't, human. You're gonna be a dolphin, that's way more fun than a merman!"
You can't believe what you are hearing. The long gray mass that extends from your butt, now full grown including tail and dorsal fins, is that of a dolphin. Within moments you see the smooth gray surface replacing your skin, your genitals integrating seamlessly into your skin. The horror really starts to strike when you see your feet and legs becoming practically muscleless and sucking back into the gray mass of your body until you practically have no legs anymore, the same happening to your hands and arms although they don't disappear completely - you can still somewhat move them now that they've turned into foot-long flippers.
You desperate try to hurl curses at the mermaids but your voice has already changed, the transformation now starting on your face. Your ears seem to go into your skull, leaving two tiny holes, and your entire face stretches to multiple times its length. One of the strangest changes is your teeth disappearing into your gums and lots of small teeth reappearing, teeth obviously suited for eating fish. Finally, your breathing system rewires itself to a hole in your neck, which is at first kinda odd as you've always breathed through your mouth and nose. After a few moments you get the hang of breathing through your neck and turn your attention to the mermaids...
"Now that you're ready to go, wanna come with us?"