Underworld demons book 1: The Laurai

The Laurai are underworld demons that are human-like and live underwater.
They are not mermaids as they don't have a tail but instead legs and flippers instead of feet, so they can still disguise themselves as human.
Ida Ingleberry is a Laurai Noble who was trained from the age of six to become an assassin, she is chosen to perform a secret mission to kidnap Vali, a boy in the human world. Given little to no information on the boy she sets out into a world she has only heard about in fairy tales. The only thing she is given is a way to track the boy and a mysterious stone that she has no clue of its use.