I want to make a new monster, using the Fey'ri from Monsters of Faerun as a base, but adapting it to aquatic environments, on the premise that a Styx Dragon demigod has crafted them in his image. The problem is, I can't make sense of the Monsters of Faerun descriptions, and I'm no good at figuring out the balance and such of abilities. I'd like it to be somewhat adaptable to a player class, and quite obviously, the fly speed should be a swim speed, but I don't know how fast, one or more of the spell-like abilities should be altered to a more Stygian Drake style, and I believe they should have a line of acid breath weapon.

Here is the fluff, if that would be helpful to anyone: Maerkolth somewhat resemble elves, but are covered in slimy scales that range from dark brown to rusty red in color. Their eyes glow a lurid yellow, and they have a long, coiling tail which splits into two long, bladed whips at the end. Their mouths are lined with sharp fangs, and their hands and feet are webbed and clawed. Their hair has colors which both range from and combine purple, blue and black in a blurry arrangement resembling the murky water that is often their home. They can survive on land just as well as beneath water. They are typically selfish and cruel, but an outsider is the more important enemy, even while feuds or house wars are still in progress.